Another approach would be to give regulars the ability to mark a post as "spam". This action would temporarily remove any recent posts by the user and block any further posts. Follow-up by a full moderator could make the block permanent.

I suspect that the spammer is "paid" when the posts show up in a search engine. Rapid banning would reduce the "half life" of the posts and make the forum much less attractive.

This approach has been used successfully on another forum that I frequent. We went from pain and complaints to virtually nothing in about a week.