Treat the Orcs as though they are an organized crime family.
If you murder a member of the Mafia, or one of the nefarious narco gangs of the modern age, do you think you'll get away with it? Will there be a bounty out on your head?
Make the party an offer they can't refuse (Movie ref: The Godfather)
A simple way to get your point across is an ambush/encounter with a large enough group of orcs from that tribe that this party can't defeat. The party is to return the contraband, pay a weregild for the death of that orc, and then they owe the current orc chief a favor. The offer they can't refuse is made by the orc tribe chief: Go get me {this Macguffin} and I call off my other bounty hunters! (Let's say he wants a prize bull owned by a lord not far away, as he wants to improve the stock of his cattle herd since his daughter needs a bigger dowry for the marriage to that other orc clan he's been dealing with...)
If they see the group of Orcs again, they'll all want their blood.
Yes, they will. You are not railroading your players if their in-game actions have consequences. As a consequence of their actions, they are now marked for vengeance or death by an orc tribe.
I don't think any persuasion roll would be good enough to dissuade a large group of Orcs from wanting to tear them limb from limb.
Is it fair to have the orcs be too angry to reason with next time they see them?
How can I prevent them from being able to bluff or persuade without the loss of player agency?
Deception/Persuasion checks with disadvantage
Yes, it's fair for the orcs to (at least at first) not be inclined to parley or listen. Attempts at persuasion/deception can, based on these circumstances, certainly be ruled to have disadvantage. You don't need to prevent the party from trying, but they are not guaranteed to succeed simply by trying to persuade or deceive a given group of orcs. You can either
- Set the DC for making a successful check very high (20+)
- and/or apply disadvantage
- During the next encounter where they try to parley with the orcs, provide the party with clues and descriptions that explicitly signal to them that "a
negotiation is not currently an option."
The DM can also decide that circumstances influence a roll in one
direction or the other and grant advantage or impose disadvantage as a
result. (PHB, Chapter 7, "Using Ability Checks")
But you don't need to roll for this. A roll is only called for when a result or an outcome is uncertain (PHB, p. 171, Using Ability Scores). The orcs are certainly upset.
The orcs might want to capture the party to bring them to justice before the orc tribe (Which might be fatal). Or, a band of orcs (calculate the encounter difficulty to hard or deadly) becomes the posse who chases the party down to capture or kill them: wanted, dead or alive! The party are now fugitives.
This isn't a railroad
This response by the orcs is the game world reacting in a rational fashion to the actions of the characters. But maybe a given orc posse can't take them out. (Back to the "offer you can't refuse" idea). The party is in no position to negotiate with the vengeful orcs (to get the tribe to stop sending hit squads out after them) without leverage of some sort. What does the party have as leverage?
- The contraband?
- Some of the orcs (captured) from the posse?
- Something else? (A unique ability the chief wants them to use on his behalf? A paladin who can, for example, cure the disease of his aunt?)
For the party to make a deal, role play the negotiations after they've had at least one group of orcs try to take them out. Put the orc chief in the role of crime/gangster boss who wants what's his, and a little bit more.
A TPK is also a valid response, but use these with great care
If luck is not with them and the orc posse defeats the party or the battle results in a TPK, that's the luck of the dice ... but you can always choose to rule that the orcs stabilize and capture/revive one of the players (or more) to be taken to the tribe and then imprisoned, enslaved, what have you.
Or the whole party gets enslaved. Now the adventure hook is: escape!
Make their mistake part of the adventure.
Ability checks aren't magic
@Ben made a comment about ability checks that is worth capturing here.
Persuasion isn't mind control. The party, or the party face, can succeed at a persuasion check without the orcs doing exactly what the party wants. For example, a success at persuasion calms the orcs down enough or muddies the waters enough that they're willing to entertain the party making amends,
rather than the orcs only pursuing bloody revenge! There are still
consequences, the party face gets to feel like they saved everyone's
bacon, and you get a story hook out of it in how the amends are made.
Best Answer
This is something you'll have to settle among yourselves.
Everyone plays D&D a bit differently, sometimes more than a bit. There are groups that go to extreme lengths to maintain a hygienic distinction between character and player knowledge to avoid "metagaming", there are groups that feel the game only improves by allowing players to leverage any knowledge they have, and there are groups that go anywhere in between. (and this is just one of the countless variations between D&D tables!)
There is no single "spirit of D&D 5e", and no single correct way to play with the character knowledge relating to the use of these checks. Words like "metagaming", "railroading", "immersion-ruining" etc are often used to disparage certain playstyles, but are seldom accompanied by justification over why these things are discouraged and in what instances.
It sounds like you and your GM both are equally convinced that your respective ways are each the one correct way to play. That is an argument we cannot resolve for you, because there isn't a single fundamentally correct way to enjoy D&D --- as far as I'm concerned both of your interpretations on what D&D is supposed to be like are equally correct.
Every way to play is correct as long as everyone around the table enjoys it --- although some ways are definitely easier for most people to enjoy (or learn to enjoy) than others. But playing two different games at the same table is very rarely enjoyable. That's why, in the future, you need to ensure you're playing the same game before these conflicts occur, and prepare to talk about what kind of game you're trying to play when they occur anyway. You will inevitably have to make concessions at some point, but that is the cost of any group activity --- it's hard to find five-ish people who are always in perfect agreement with you.
A popular tool for these discussions is known as the Same Page Tool. It looks like a questionnaire, but should be treated as discussion prompts for the entire group (every time I've used it as a poll has ended in a bunch of unresolved conflicts that emerged later in gameplay). It's also not something you should vote on --- if people disagree, the best course action is to talk out the disagreement and see if either side is convinced. If the point isn't important to you, concede. If you can't reach an agreement over an issue you deeply care about, look out for a game group that matches your tastes better.
Session Zero is a good place to have these discussions. Even though its name might imply that it happens before the first session, it's not too late to have a session zero now, to realign with the rest of your group about what kind of game you want to play. In fact, I try to personally arrange some "mini-session zeros" with all actual sessions, for discussing what worked, what didn't, and where people want to see the campaign develop.
Summa summarum, this is a problem between you and your GM, your respective expectations of the game not aligning, not a problem in either of your interpretations. If you don't want to let go of your interpretation, and the GM not of theirs, you should find someone else to play with and hopefully help yourself and them discuss your expectations of the game in terms that allow you to avoid or resolve such unpleasant conflicts in the future.