To delete the subtitles, only the person who created them or the Channel Manager can do it.
But the Channel Manager can only delete a whole episode subtitles for a language not just a part.
The person who created the subtitles has to go on it, one by one, deleting the whole sentence written on the segment to delete it.
As for what you should do when someone translates without your consent as Moderator, you did the right thing, I think.
You should first contact the subtitler who did that, ask her/him not to do that again and to delete her/his subtitles.
When the subtitler understands and deletes her/his subtitles, there is no problem, but if not you have to contact the Channel Manager to see if she/he can delete the subs precising in which language and which episode.
And after, it's up to you seeing what you want to do to avoid this kind of situation.
Sometimes, when one subber does this, I prefer to lock all episodes (when it's not already locked) right after to be sure this mistake will not recur (after the subber deleted her/his subtitles).
Generally, I know well the subbers I have in my teams, and if not I always send them a message before the begining of the drama to let them know how I work and what they don't have the right to do.
Like this, I generally have no problem.
I hope my answer can help you.