Protecting from unnatural links at present is pretty difficult thing to do in the present seo climate which is why google are trying to knock out a type of "link disavow" tool to help webmasters.
Lets say you have 10,000 posts on a forum and you use the sig link to promote a site (especially a new site), google spiders may suddenly find lets say 1000 or so links from the same domain all pointing to your site and with the same keyphrase. They may see this as unnatural.
If you get some of these cheap seo teams to run a promotion campaign for $5 or so, you may get a lot of links dropped in areas that google has devalued and this may also cost your site.
There is no actual way of protecting from them at the moment and the only thing you can do is to do your best to keep your site and content as individual as you can.
Though that said, don't go heavily promoting a key phrase as this can/will come across as unnatural, especially since the Penguin algo update.