If plagiarism offenders are banned, I wonder why scammers who usually do a lot more damage are spared.
See the following quote from mprep's Unofficial Rules thread:
Q: Why haven't you banned <insert scammer username here> who is an obvious scammer?
A: Possible (or real, not for me to decide) scams are not moderated to prevent moderator abuse. If we start picking out which ones we call "scammers" and ban, we would make a lot of decisions based on biased opinions.
It would also give off an illusion of safety. If there was a culture of banning scams, then users may operate under the false assumption that any offer or service they come across on this forum is legit, when in fact it could simply be a scam which hadn't been caught yet. Not to mention there just simply isn't enough moderator time to examine the evidence for every scam accusation.