Can you check IP numbers? Of course, this is also only a partial solution. Some providers give to clients a new IP number every day ...

In my opinion, there are different solutions:
1) Try to block them through their IPs or something similar (I'm not able to give any advice in particular how it can be done);
2) Block those users as soon as you recognize them with their new accounts and mark red and release all their contribution to let other contributors adopt and correct their sentences, and delete those of them which are insulting or couldn't be understood and corrected.
3) If they only contribute "bad" (grammatically incorrect or unnatural sentences), but not insult nor troll anyone, you can let them contribute "as is", but mark it red, and encourage other contributors check and correct their sentences, and give corpus maintainers and advanced contributors the right to remove red colour from their native-like language sentences.

I think, trolls and vandals are something that any open sourse project have to deal with, it's an inevitable evil, but also it's important to not turn people into vandals with some unfriendly attitude. And it would be helpful also encourage admins and corpus maintainers be more friendly to non-native contributors and help them to make their contribution trustworthed.

Maybe instead of suspending such contibutors they could be redirected to a fake instance of Tatoeba without their knowledge. Dev.tatoeba for deviants. In contrast to the actual dev.Tatoeba, this instance needs to be constantly fed with contributions and comments from the “real” Tateoba, without affecting the dev-contributions. So nobody could be sure which Tatoeba they are actually in. All they would notice is that they are being ignored. Well, not really, other devs can of course translate and comment their entries. Maybe admins can switch to dev.Tatoeba to give some feedback from time to time (** The 2 Ways You Can Be the Most Helpful ** and so on) to make the thing more credible.

Daha Türkçe bilmeyenler, Türkçe cümlelerine çevirenleri suçluyorlar. Onları haksız yere yargılamayı kesin artık.

As a Turkish speaker, I'm well aware of the problem.

The current methods for suspending accounts and deleting sentences on Tatoeba are not efficient against troll activity. For instance, when an account is suspended, the sentences and comments posted with it still remain. Even if a sentence is deleted, it can still be seen on the logs section of the related account. The suspended accounts still appear on the member list page as well. Hence, suspending an account actually often turns into an intermission. If there were a more aggressive procedure against troll accounts that deleted them as if they never existed, it would be more demotivating than just deactivating them, I believe.

Temporarily blocking the IP address ranges used by the most notorious trolls may also help them give up their obsession, but of course, that would only be a makeshift precaution.

Unfortunately, the Turkish trolls are not the only nor the biggest problem for the Turkish corpus, I think. Along with the bad sentences added by them, there are also too many literal translations, clumsy and less-than-natural sentences in the Turkish corpus, mostly contributed by a quantity-focused CM. It's difficult for a non-native speaker to notice that, though.