I've been here since 2014, and whenever there's a scam I'm always glad that the ANN threads are not deleted. They serve as a good teaching source. Sometimes I go back and go through the "Real Stack Coin," scam and read on how excited the people were until the dev announced that it was scam, rubbing it in with a party gif.
I don't see any need to ban a scammer, because then they'll create another account anyway. And I doubt anyone will trust them again. The guy that you talked about has a -241 rating.
You have a very valid point about it being a good teaching source. Also I love history - censorship can kill some history.
Also google indexes bitcointalk.org quite well. So if you make a scam accusation or post in a thread that is not self moderated then it will get indexed on google. "XYZ ICO is a scam" indexes quite well on google and may warn people about it.
I sometimes warn on a self moderated thread that if they delete my genuine question I will open a specific thread about it and they won't be able to moderate it. Often that leads to either good dialogue or a scam accusation thread.