Because banning him would give him an excuse not to fill the orders he's long over due on?
He only shows up once in a while and doesn't cause any new trouble and hasn't made any new offers to take money for stuff (not that he'd be allowed to). He does seem to be making some sort of attempt to fulfill his owes. It may be just occational BS to stay on the map or he might actually be trying. Who knows?
Like I said; I'm not holding my breath but if the sonic screwdriver he's owed me for some time shows up in the mail I'll still be happy to have it in my collection.