I don't think you need to search very far -
See these quotes from the warlock class description (Complete Arcane P. 5-6)
WARLOCK
Born of a supernatural bloodline, a warlock seeks to master the perilous magic that suffuses his soul [...]
[...]
Adventures: Many warlocks are champions of dark and chaotic powers. Long ago, they (or in some cases, their ancestors) forged grim pacts with dangerous extraplanar powers, trading portions of their souls in exchange for supernatural power. While many warlocks have turned away from evil [...] they are still chained by the old packs through which they acquired their powers [...]
[...]
Background: Warlocks are born, not made. Some are descendants of people who trafficked with demons and devils long ago. Some seek out the dark powers as youths, [...] but a few blameless individuals are simply marked out by the supernatural forces as conduits and tools.
The exact nature of the warlock's origin is up to the player to decide;
[...]
In fact, many warlocks are created by nonevil powers - wild or fey forces that can be every bit as dangerous as demons or devils.
(All emphasis mine)
AFIK, This is the source for the definition of warlock class in D&D 3.5, with Complete Mage expanding on it, including providing rules for warlock of non-fiendish origin, among other things.
At any rate, three things are stated here which are relevant to your question:
Not all warlocks have actively made a pact with a supernatural power
The exact nature of the warlock's origin is up to the player to decide.
The description provides three alternatives:
- Your warlock made a pact with a "supernatural power" himself.
- Your warlock's ancestors made the pact.
- Your warlock is one of the "[few blameless individuals who] are simply marked out by the supernatural forces". He didn't choose it - they've chosen him...
So, if your warlock's is a "type 2" or "type 3" - he didn't make the pact himself.
Not all warlocks have a fiendish origin
In fact, many warlocks are created by nonevil powers - wild or fey forces that can be every bit as dangerous as demons or devils.
So, your warlock may have an elemental lord, powerful fey or even a slaadi or celestial as the source behind his power.
Every warlock owes his powers to some extraplanar or supernatural source creature
Even if your warlord didn't make a pact himself, and even if his source is not fiendish, there's no such thing as a "self empowered" warlock.
While many warlocks have turned away from evil [...] they are still chained by the old packs through which they acquired their powers.
While this have very little effect from a mechanical perspective, the warlock class is defined with this "built-in" narrative conflict or impediment - he has a would-be master - Some powerful extraplanar entity which have plans or invested interest in the warlock.
Since this entity isn't as mighty as a deity, the warlock doesn't lose his powers if he deviates from his would-be master's plans, and he may even defy him outright. But, if you play a warlock, it comes as a given that you should expect some supernatural meddling in your affairs - maybe the warlock is merely observed, maybe he is not that important to that entity, and maybe it'll notice him only after he attracts its attention (by going with / against its interests or by simply becoming powerful enough to serve it as a useful tool).
So, while you are technically correct in that not all warlocks personally made a pact with a fiend, you should still work with your DM to define the source of your powers. At the very least, provide the type of that source(1), i.e. whether your warlock powers are fiendish / elemental / fey / celestial / etc in nature. But I'd recommend that you describe the source as an entity, with schemes and goals of its own(2) - you'll be missing out on role-play opportunities, plot-hooks and character depth if you settle on a generic "its in my bloodline" origin.
Finally
As a side note, you state in the question that this group has a certain way of doing things, so I wonder how constructive can "smacking them with a sourcebook" be. Clearly, a healthy gaming group should be open to debate regarding rules, and perhaps even settings and campaign world elements (to some degree), but in the end, the DM calls the shots, especially when it comes to the narrative setting. So unless you can convince your DM and group to go along with your interpretation, you may have to play with them on their terms, or leave that group - no matter how many sourcebooks and splatbooks are on your side...
(1) I don't know how much leeway a DM has when running NWN2, but at a tabletop game there could be a vast difference in the way an invocation is described, and how NPCs react to such manifestations, depending on the warlock's origin - for example, a good cleric may instinctively oppose anyone who displays demonic powers, while being indifferent to someone using the same powers flavored as fey or elemental.
(2) The specifics of these schemes probably could be left to the DM to use/abuse without sharing the details and ruining the surprise for you...
Primary Consideration
Due to the controversial nature that this specific problem represents, it takes a mature player and a mature DM to handle this specific scenario well. Ensure that the player and DM have a conversation about something of the magnitude of power revocation BEFORE it is implemented in play...
If you are the DM and this is established as part of your world, ensure that players know this up front if they are considering playing a warlock (or any other class likely to have powers revoked for any reason).
If you are a player and you want this to either be highly probable, talk with your DM. It can create an intriguing story.
Already covered
RAW, there isn't any explicit text that covers a warlock's powers being stripped. Also, there is no class that has rules text covering a loss of powers, save the Paladin whose powers change form when he becomes an Oathbreaker. (DMG p. 97)
Lore from stories associated with various pact-style magic demonstrates and sets precedent that it could be a good story hook, allowing for a very interesting story line that leads to all the things that were mentioned in the original question.
Also Consider
The power belongs to the entity to dole out. If they granted it, it is likely true that they could take it away. This is more of a reference to old literature, to what makes sense, and to what would be fun with the game you and your DM seem to be trying to build.
The rules do not state the specific pact, this is the part that cannot be stressed enough. The power that they currently have should mostly be considered as payment for services rendered. If they are not completely paid for (ongoing payment, for example), then try to avoid stripping a lot of their power from them. Make it minimal, but noticeable. Lower spell slot levels by one levels as that part hasn't been paid in full, if that helps the story line, but do not completely cripple the character.
As for the specific scenario posted in the question:
The devil is highly unlikely to strip power from someone if it is part of a contract. To do so is highly unorthodox for a devil, and would be looked at even more poorly than an upstart servant that the devil couldn't control. The devils live by their contracts, and although adding loop-holes may be favored, rescinding an agreement is not. If a devil were to rescind the power of one of their warlocks, it is likely that another entity would try to mock them by taking the contract instead. This is even more true of a relatively powerful warlock. That said, a devil is also very likely to add a hidden clause that prevents the warlock from using their powers against the devil, with revocation of powers being either temporary or permanent, depending on the devil, the warlock, and the devil's disposition at the time.
They will follow the letter of the contract completely, but don't care about the spirit of the contract. If the devil is powerful enough to grant powers as a patron, they are likely intelligent enough to be more careful about the contract, though a particularly savvy and/or intelligent character could manipulate them in to a contract that is written to the benefit of the character more than the patron (protecting themselves from abandoning the patron, for example).
If you really want to do it
The best way to add the possibility of power revocation for story-line purposes is to ensure that it is an active part of the party's story (as opposed to a passive one). Allow the party to try to stop the powers from getting revoked, or have another patron (possibly a more sinister one) inform the warlock that they are going to lose their powers and offer to be a surrogate patron.
Do something that directly involves the warlock in whether they lose or retain their powers.
In short
Is it possible? Rules don't state that it is, but this is something a DM could easily say yes to with good justification to back it up based on game world.
Should the DM take this path? Probably not, or at least not seriously. If he does, then he should offer a work-around. Don't strip power from a character/player without offering a way of obtaining it again. Don't make the game less fun.
Best Answer
The Soul Is Not Required Collateral
(But it Provides Good Leverage for the Fiend)
In the MM, page 51, a demon is described as
As written in the Player's Handbook, the Warlock's pact does not necessarily involve the Warlock's soul for the transaction, or the relationship, that leads to powers being bestowed by the Fiend as patron. The discussion on PHB 108 and 109 declares that the patrons
Furthermore, the Fiend desires
Nowhere is the bargaining described explicitly as the soul being traded for the powers. There are plenty of other ways to destroy or corrupt a Warlock. Consigning the soul to the Abyss, or making the Warlock wear a metal bikini while chained to the Demon for eternity is one way it could play out, but it isn't a necessary condition.
Were one to try and raise a Warlock whose soul was the collateral, the Fiend
May not wish to release that soul,
May consign the soul to the Abyss and then no longer cares
On point 3 ...
On point 2 ...
The options are only limited by the imagination of the DM. So, work with your DM.
If you are the DM, figure out a bargain, up to and including the soul as collateral for power, and then apply leverage. That's what demons do. (The trope is as long standing as the idea of making a deal with the devil/demon for earthly power. )
For example, a Warlock whose contract is with a Fiend, per page 105, may lead a cult. In that case, the Fiend may demand/require that the Warlock feed the Fiend the souls of cult members, offering them up in some exotic or dark ritual. The Warlock's soul is not at risk so long as the quarterly payments are made. Miss a payment and you get a collection agency showing up at first, a few lower level demons, and then it gets more interesting.
As to "are there any on-line" sub question, you can take a look at reddit or a pact example like this. The favored beer in Hell is apparently Falstaff. (Taken from Marlowe's Play Dr Faustus, wherein Falstaff and Mephistopheles work a deal ...)