At one point my party cleric found a magic item that allowed him to pull anything smaller than the item out of it, like a Star Trek replicator. Now nothing is a challenge for him anymore. How would I make the item less powerful without just saying "your item is now less powerful" and breaking the immersion of the game by violating continuity?
[RPG] How to depower overpowered magic items without breaking immersion
dnd-5emagic-items
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The DM controls treasure…
The Dungeon Master's Guide (1979) on Placement of Monetary Treasure says
All monsters would not and should not possess treasure! The treasure types given in the Monster Manual are the optimums and ore meant to consider the maximum number of creatures guarding them. Many of the monsters shown as possessing some form of wealth are quite unlikely to have any at all. This is not a contradiction in the rules, but an admonition to the DM not to give away too much! Any treasure possessed by weak, low-level monsters will be trifling compared to what numbers of stronger monsters might guard. So in distributing wealth amongst the creatures which inhabit the upper levels of dungeons/dungeon-like areas, as well as for petty monsters dwelling in small numbers in the wilderness, assign it accordingly. The bulk of such treasure will be copper pieces and silver. Perhaps there will be a bit of ivory or a cunningly-crafted item worth a few gold pieces. (91-2)
The section then details how the DM places monetary treasure. This is followed by a section describing the placement of magic items (92-3). Neither section encourages slavish adherence to random treasure table rolls and, on the contrary, recommends the DM place (not roll) monetary and magical treasure based on the creatures encountered and the player characters themselves.
So the DM decides what treasure exists not the dice, and if the DM doesn't like a rolled result, the DM rejects that result and either rolls again or just picks something. Even in AD&D, a DM shouldn't feel as though he's, like, cheating by rerolling or even picking what's best for that campaign instead of inserting something the DM knows is bad for campaign.
Thus, while the dice might've dictated that a ring of three wishes is somewhere in the dungeon, that doesn't mean the DM then must put it there or else the DM's doing it wrong. In fact, I suspect that Gygax would likely put forth that DM is doing his players a favor by excluding from the 2nd-level dungeon a ring of three wishes: a treasure like that at low levels skews players' expectations, giving them less to look forward to at higher levels when such treasures can be rightfully earned instead of accidentally rolled.
The Monster Manual (1978) in its description of treasure type further explains that the monster's treasure type entry means that such
treasures are only found in the lairs of monsters[, and] it must be stated that treasure types are based upon the occurrence of a mean number of monsters as indicated by the number appearing and adjustments detailed in the explanatory material particular to the monster in question. Adjustment downwards should always be made for instances where a few monsters are encountered. Similarly, a minor adjustment upwards might be called for if the actual number of monsters encountered is greatly in excess of the mean. The use of treasure type to determine the treasure guarded by a creature in a dungeon is not generally recommended. (5)
So this means the level 1 PCs upon defeating the 1–4 giant ants (see the DMG's Dungeon Random Monster Tables for Monster Level 1 on 175) should not get all or even the bulk of the giant ants' Q×3 and S treasure types but, instead, only a tiny portion—if any!—of that treasure, the Number Appearing entry of giant ants being 1–100, and even that tiny portion of giant anty treasure assuming the PCs defeat the giant ants in their (probably less than expansive) lair.
(If the DM doesn't mind mixing his dnd-bx material with his AD&D material, this DM recommends the Monster & Treasure Assortment: Sets One–Three: Levels One–Nine (1980) for a multitude of examples of reasonable lower-level treasures suitable for a party that's defeated, for example, some not-in-their-lair, less-than-the-listed-mean wandering monsters. The tables, if so desired, can also be used pretty much as-is, needing hardly any conversion.)
…But I understand wanting to roll dice and stick with the results
So you rolled a ring of three wishes and feel like, because it was rolled, it should be in the dungeon, darn it! That's cool. Make the ring almost inaccessible, nearly impossible to find except through great ingenuity, skill, or luck (maybe the ring's surrounded by puzzles and traps as the dungeon's centerpiece or made of glass and at the bottom of an underground lake). Or, perhaps even better, somewhere in the dungeon there's a clue that leads to another dungeon where the ring can be found. Or you can just give the PCs such a ring and see what happens; remember, these are AD&D wishes, after all.
Pathfinder's rules for damaging objects says:
Attended (Held/Wielded etc.) Items: Unless the descriptive text for a spell (or attack) specifies otherwise, all items carried or worn by a creature are assumed to survive a magical attack. If a creature rolls a natural 1 on its saving throw against the effect, however, an exposed item is harmed (if the attack can harm objects).
...
So the worst that could happen, is that one item might get damaged or destroyed.
And that's only on a spell that allows saving throws (which Shocking Grasp doesn't), and only if the creature rolls a 1 on their save.
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Best Answer
The great part about magical items is that they can be damaged or weaken by quite a few factors.
Several things can be done in-story to weaken this item.
Add a slow, yet noticeable curse. For example, if he uses the bag too many times, the items that are pulled from there are red hot, burning his hand when he pulls them out. The more he uses it, the bigger the damage die. Or, you could start having him roll on random tables, and the more the bag is used the less likely it is that the desired item will be pulled out. If it acts slowly, it could explain why it has so far gone unnoticed.
Add some external, magic-disturbing force. If your characters enter an enter affected by Wild magic surges, who's to tell what can happen to spells or magical items. This could give context to why the item's power fades slightly. Also, entering an antimagic field can disrupt the power of some of the most legendary items, why not this bag?
Have someone steal the real bag and replace it with a fake, weaker version. Does he hold this bag close to him, but brags about it in crowded taverns? A thief with sticky hands might decide to trick him into thinking a false bag is the real thing, while stealing his actual bag.
Can the bag be slightly damaged? If the bag by some unfortunate accident is torn a bit or some water damages its lining, perhaps it only works on some attempts instead of every time he uses it. Maybe the selection of items is reduced, as the part of the bag which created those items has been damaged.
There are several other options, but at the end of the day if nothing seems to be satisfactory, simply talk to the player about this. He might be able to offer suggestions that will allow the preservation of his immersion, which is the point of all this.