I agree with your point about additional warnings - these spammers don't read the rules, don't read the stickies, don't even read the posts in the thread they are replying to. They certainly won't bother to read any other warnings or rules, regardless of where you plaster them.

However, having 50% of the threads in Meta being ban appeals from plagiarizers is a real headache to sort through, not to mention a waste of everybody's time continually pointing out to them that they will not be unbanned. If we are saying it is too time consuming to state the exact reason for every ban, could we at least update the ban message to state something along the lines of "Bans for plagiarism will never be repealed. Do not bother trying"?