As far as I can recall, a banned account is not locked, so it is still possible to log into the account but not create, move, alter posts, nor send PMs from the account.

People can edit or delete (not all, depending on the board) their posts before they are banned, so if they “feel” they are in the process of being caught plagiarizing or whatnot, they could alter the evidences. That is why some people quote the plagiarized post and/or archive them to an external site for reference. Nevertheless, I figure internally there is some way to track these changes on the live DB (admins/mods?).