Yes, it's kind tough and that is why I think Alex is having trouble figuring out a way to sort the real from the fake.
The way the spambots are working is some dudes server, russia, nigeria, whatever, has a script that is running these spider crawlers across the web and auto registering, and posting. Sometimes they might be manually registered by the crappier scripts. Almost always the end target is always volume (more sites the better) than quality, which is how it becomes easy to spot or they give up easily after getting banned.
I have (had) a solution, it's a bit extreme but I think it may be successful. Set new members under maybe, ~5 posts to be unable to link to another site, or post a picture or image. It hopefully should take care of a majority of spam because they'll just loop their posts with generated links and find out they can't post it, the cycle keeps going.
Unless, they are smart enough to bypass the blocks and not include links or images. In that case it's going to take more manual work like us around to report it.
Most newcomers don't seem to post links or pictures very often if at all, so this might kill two birds with one stone.