George
Although this isn't a 'fully moderated forum' in the strict sense of the term there are enough of us around at most times of the day or night to squash them as soon as we see them.
Unless this other forum to which you refer is more proactive in stopping them it will eventually become unusable as all the other forum spammers in the world will see how easy it is to get their adverts on board and thus get a Google listing. Very short sighted.
As Brian says, Caroline spots most of them. We all have a laugh at some of the tricks these people get up to. The easist ones to spot are the ones who's first ever post on the forum starts off something like 'I've just been on to Google looking for [Nike trainers], [mobile phones], [cheap web hosting] and I found this
stolen.goods@china.com I think it's great!!'