Some other forums require admin approval of a registration before posting privileges are established. I am not sure what you would check to determine if they are a spammer or not (maybe their profile entries?). Perhaps restricting new members in some way such as only allowing 1 or 2 posts prior to admin approval, or not allowing posting to proposed new topics or other less restricted forums such as "suggestions and questions" or "coffee house."
I don't know, maybe it doesn't happen enough to be a concern but that was a helluva mess this morning - 20 pages of spam!!! Thanks for doing all the work it took to clean up!
I also can't understand why this spammer would think that blasting the forum with hundreds of posts that we can't even read would tempt us to use whatever it was it he was selling

HBD
Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for... I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca
"Nothing is easier than to persuade people who want to be persuaded and already believe." - another Petrarca gem.
Ignorance is a most formidable opponent rivaled only by arrogance; but when the two join forces, one is all but invincible.