you cant perm ban ip's due to people using tor etc
You can't perma-ban them, but if Bitcointalk begins to IP ban, then users will have to begin paying fees to sign up, which might start fending off some of the scammers.
I like what others have suggested. Sites like Anandtech and [H] require people to have x number of posts before posting a classified. Overclock.net is a bit tougher on newbies and classifieds, and require 35 "rep" before users can post a classified. Rep is earned from posting useful posts, sort of like upvotes on Reddit, but they seem rarer to get.
Maybe a reputation system similar to OCN, and requiring a reputation point count >= 15 would cut down on these scams. I know it would take a lot of effort to implement something like this, and some people wouldn't like it, but it might be a decent solution.