From time to time, cretins drop by the forum to publish some of their spam, usually advertising links to other web sites. They do this because it’s profitable – because it does work for a tiny minority of people who respond to such advertising. So the best cure is: never respond to spam.
Go out of your way to avoid spamvertised products, and if you get the chance, tell the seller you’re not buying from him because of the way he advertises. (Please notice that the seller isn’t usually the spammer, and most spammer accounts are throwaways, so don’t respond directly to the spammer.)
As for the posts themselves, down at the end of the post you’ll see a “report” button. Instead of replying to the post, just click that “report” button to tell a moderator about it, and they’ll delete the post (and very likely the poster too.) But there really isn’t much point in responding, since the whole thread will vanish as soon as a moderator spots it.