How about a simple system that requires a newcomer with less than (say) 5 posts in other Threads to use the title "Hi, this is my first Thread" when s/he starts her/his first Thread.
That title would be useless to someone trying to advertise something. And the newcomer could edit the title later - after s/he has made 5 or more posts.
It would also allow moderators (or, better, some Forum software) to detect new Threads with greatest risk of being Spam.
...R