InfinitusEquitas is right. It's not going to be a big issue. If the advertiser is smart, and isn't just buying ad space monthly (which I suspect is what most do here), he/she will require a click-through rate, rather than rely on ad impressions (paying each time the ad is displayed). I doubt most people here have clicked on the ads, so they're not helping with click-through rates.
Also, many forums will outright remove ads for members who sign-up for an account. After all, the members usually "know better" when it comes to places to shop, or will just buy from other members, not from stores featured in the ads.
Finally, many of the ads can't be blocked by Adblock Plus. I've got ABP on, a HOSTS file to deny certain sites, and also have some frames blocked (like the ones at the bottom of the site). There are still ads, but the annoying ones are gone. Regardless, I've never actually clicked on the ads--and sure as hell never clicked the Wicked Lasers ad, like appears right below the quick reply box.
