Thanks for asking, Giacomo, that's a long time problem which cannot be easily solved.
Often most is up to your judgement , as Mark says there are obvious spam threads, but many aren't so clearly definable.
The cases in which I don't consider it spam is when there are obvious reasons as to why what the user posted should stay on the forum. If it is useful for the community, with chances of discussion, I tend to let it through.
Also, try to take into account Paolo and the site's interests, if it mentions a rival website it's better to delete.
It varies also depending on the user, the same link posted by you or Mark or Villa, and in the first post of a member, is usually treated differently.
It's important that posts are for the sake of the forum, and not for personal earning of the members, and generally you can judge by this. Naturally that's the reason for the difference of treatment, as you guys gave so much to the community that it can't be thought you're here to exploit it