Pretty sure they have rules for this in 3.5. I know they do for Paladins that get off alignment (it's also how you make some prestige classes to create an "evil" paladin or death knight or whatever it was called).

With Warlocks, I feel like the punishment could be anything, as the devil's contract would likely stipulate exactly what happens if you fail to obey the patron. Maybe you lose your powers. Maybe they draft you into the Blood War. Maybe they just take your ass to the Hells.

In general, you don't want a player to loser their powers. But if you can turn the threat of losing their powers into a plotline, then that's awesome. Warlocks losing powers for breaking contracts is bad. Warlocks having to fend off eldritch repo folks for breaking contracts is good.