I feel bad for these warlocks who entered such bad deals. My infernal warlock, Johnny the folk-hero, won his warlock powers from a fiend in a fiddle playing contest. The fiend lost and had no choice but to give Johnny a sliver of power.
Sure, he could kill Johnny outright but word would eventually get around and fewer and fewer people would be willing to enter into these contest.
A DM can tell me No Warlocks and that is fine, he can even drop rocks on my characters head untill he dies and I would understand that is his perogative. But to intentionally throw something in the game to strip my character of his class? No, just drop the rocks and call it a day.
Now if he routinely targets the wizards spell books and breaks the fighters weapons......at least he is playing fair.