When someone is deleting posts on a self moderated thread (and they are scamming) then people often are going to post their warnings a second time (and subsequent times as necessary).
This would really not work anyway because people could just create an additional account when they get banned from posting in it
But trolls also do the same. People are already sceptical of users who self-mod threads anyway and they will be even more so when they're banning legit users. You could also say banning doesn't work because they can just create a new account. They usually get bored of doing so pretty quick.