Do you think they'll come back if I ask real nice?
Fed up with constantly deleting spam I finally did what I didn't want to do. When a user first signs up, all of their posts are moderated until I determine that they are not a soulless spammer. Moderated means that I can see them, but no one else can unless I validate the post, which obviously I would be disinclined to do if the primary purpose of said post is to sell ugg boots.
The big problem of course is that this will dissuade new users from joining the conversation, but since we haven't seen one of those in a while I went ahead and pulled the trigger.