The more active & bigger the forum gets the more spammers you get.
The spammers use different cloaking methods - hiding the links discreetly, so that you won't spot it if you do not look carefully, perhaps it is in the upper right corner or another trick used by human spammers - they register make a post then come back two weeks later & edit their post with spam links.
Automated spammers - you can ban the suckers as fast as you can but, they come back!
They are robotized scripts that look for weaknesses in forum platforms (there are not that many and the vulnerabilities are known) to automatically do this kind of thing....
There is a new piece of software available for forums to help identify and ban spammers. It works by working with a global blacklist generated from lots of forums around the world.
Having active staff is the best method to keep the forum clean!