RPGMemes

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.