Any user who is banned for plagiarism gets a message explaining the reasons for the ban, as @hilariousetc commented. I do not know exactly the text of the message, but it should have something saying that the ban is definitive and that there is no way to appeal.
Theymos has recently changed the ban message to something along the lines of
you have likely been banned for plagiarism/copy and pasting. The issue is is that forum accounts are like gold dust or their holy cash cow to the type of people who get banned for plagiarism so they're not just going to let them go without a fight and will do and say anything out of desperation if they can if they think they've got even the slightest chance of getting their accounts back. A criminal would do the same after being accused of theft but until they see the evidence they'll try anything they can to wriggle out of something and avoid punishment and that's why users request proof even when they know what they've done because they're hoping we just can't find it or have a made a mistake or whatever.
I think after explaining which post the user plagiarized, the topic should be blocked. And only moderators or administrators post on it if the situation requires it. Any other communication should be made by the appeal email. I'm not saying that I agree with this rule of ban forever who plagiarized. Neither do I disagree with the right of defense of those who received a ban. I just do not believe that the best way to appeal against such punishment is with these topics.
I wouldn't be against some sort of appeals sub board and maybe only staff can posts in there, but locking thread is a tricky issue. Who gets to say when the issue is closed? True, ban appeals are often an open and shut case but many times users still want to protest. Even if we locked the thread there's nothing stopping them from just opening another which they likely would if they feel the issue is unresolved.
Any user who is banned for plagiarism gets a message explaining the reasons for the ban, as @hilariousetc commented.
Where did he write that?
I didn't. Temp bans state the reason and as I said above theymos has recently changed the permaban message to include that they were likely banned for plagiarism but it doesn't explicitly state what they were banned for.