As written, I don't see any (game-mechanical) consequences for a Warlock accepting powers froma Patron and then using said powers in a manner that opposes the desires of his/her Patron, i.e. a Warlock with the Fiend Pact from an Arch-Devil using his powers to fight the Devil's own forces. Can a Patron revoke the powers it's granted to a Warlock? Should a Patron be able to do this? In previous editions, deities could strip a Cleric of some or all of his spells if the Cleric made his god mad - aren't Warlocks basically the Arcane equivalent of Clerics? Of course, in 5th Edition Clerics don't seem to be subject to this rule anymore, either...
It's a very good question, and I strongly believe that the game itself does
not dictate every DM to handle this in one official way by the RAW.
This is my typical way of handling the issue:
Warlock: the details of the pact are up to the player herself, so there are no obligations towards the patron unless the player has decided that there should be. That's because the nature of the pact can be very variable. In fact, when presenting the Warlock class, I pretty much always say that the original class concept was in the "Faust-ian pact" between a mortal and a fiend, for example selling her soul in exchange for great powers; to the fiend it's irrelevant what the warlock does with her powers (and in fact I always quote this case as an example of how a character can be
good-aligned while being a Fiend Warlock at the same time), because the patron still gets her part of the deal fulfilled when the character dies.
Cleric: unlike a Warlock who is motivated by the
benefits of the pact, a Cleric is motivated by the ethics/morals/religion itself. This is very different, and a Cleric who betrays her ethos essentially betrays herself. It should be the Cleric herself to feel guilty if not living up to her own religion and be motivated to atone as soon as possible, if you want to have a consistently well-roleplayed Cleric. In the rare case the Cleric voluntarily strays away from the chosen ethos, I would let her lose her divine powers. But if such case happened, then I would probably suggest for the Cleric to instead switch to a different religion, and have her powers adapted. I think this way of handling the issue for Clerics works fine both when you narrate the Cleric's power to emanate directly from her deity (i.e. the traditional narrative) or when you narrate it as emanating from the Cleric's inner faith (in which case, a Cleric who consciously breaks her own vows stops believing in herself, but can possibly start believing in a new self).
Druid: essentially I treat Druids more similarly to Wizards than Clerics, despite the fact that they have a religious narrative, so the "losing your power if not keeping up with your faith" is not much of an issue at all.