Since when spam needs to be explicitly forbidden?
It goes without saying, in my opinion. This is a Q&A site, there are advertising possibilities on it, it's pretty clear that questions and answers are not the places for people to advertise their products.
It would be also useless to make such a line in the FAQ. It won't prevent any spammer from posting their stuff. I don't think there is a need to be "fair to them".
Maybe your situation differs from Super User in that you get more "corporate" advertisement, but the effect remains the same. There is link for advertisers at the bottom. If they just want to spam their product links, it shows that they don't care and just want some free exposure.
About spam in general, I have found mostly two kinds :
the robots: the ones who have a site/product to promote, often of bad quality, and will simply answer to all related questions, even the ones which were answered and sleeping for 6 months, with their link and a basic description. No discussion to have with these ones, from what I experimented. You can try to leave a comment to see if they react, they never will. On SU, we used to simply destroy them on sight. Now we do something more valuable and aesthetical: we fuse them into one inert monster/piece of art.
the traveling salesmen: they actually work(ed) on the product they are advertising, and are acting like a regular commercial. Answers will be more adapted to the questions, and they will sometimes post actually useful answers using their product. This kind is more likely to actually answer to comment. It's ok to give your own product as an answer once in a while, as long as your contribute in other ways. For the ones who are actually only writing about their product, they should be reminded of the "advertising info" link at the bottom, and warned that they are on the thin line to the "robot" behavior, and face the same fate.