I mean, are they doing something against the Blizzard ToS? Are they using hacks to have 5 copies of Possessed Villager?
What's that? They're only using a deck that you personally don't like? Well shit, best ban them right now!
Okay, massive amounts of sarcasm aside, what else could Warlock do? Sure, there's Discardlock, but not only is that archtype extremely limited in design (There are few cards that cause Discard, and fewer that benefit from it, like Silverware Golem and Malchazzar's Imp.) but it's also very unreliable, due to how the discard mechanic works. It's a fun deck to use, but not very viable without fusing it with Zoo. Renolock is also a decent archtype, but it also uses a lot of the same cards that normal Zoolock uses, such as Imp Gang Boss and Possessed Villager.
Is Zoolock annoying to face? Yes, of course. But it works. Just like Aggro Shaman before the recent patch neutered it to hell, Zoolock is a relatively cheap, effective deck. Granted, you still need to know what you're doing for it to work properly (Like any deck, really.) and there's still ways to beat it.
Guess what I'm saying is while yes, it's annoying... Banning the people who use it is an overreaction to the Nth degree, friend.