To be honest, I don't think that this user gonna leave this forum just because he got banned. What he is probably gonna do is create new account, and continue with business as usual and if he has any brain, he will avoid plagiarism.

That's probably the main reason why I prefer to warn newbies instead just reporting them for plagiarism as if their goal is to profit from this forum in one way or another, they will just create another account and continue like nothing happened, while with warning there is a chance (albeit small) that he will edit the post and learn something.