Only admins can block a user from uploading, but this is a pretty rigorous method.
I understand what you are saying about such credits. Credits make total sense, it's the only reward users get for their unpaid work, but I agree it shouldn't be too much and mutilate the subtitles. Still, I think this is not enough reason to ban the user.
I would recommend to communicate with this user. Point out that credits are totally fine, but shouldn't interfere too much with the movie/episode and are normally limited to maximum two times a single line of text. If you kindly explain, that user may change his ways.
Meanwhile, you could just avoid using the subs of this user.