Here's a recent rant from search journalist, Danny Sullivan, regarding link spam. This is a good read for anyone involved with SEO.
"...the core problem is that the web has people who think nothing of vandalizing other web sites. That’s what link spamming is. You’re not adding value to a site. You’re simply spray painting garbage on someone else’s property, for your own personal benefit. You have no manners. You have no morals. You ought to be ashamed." - read more.
Community sites, such as discussion forums, social networking platforms, and blogs can use automated filters and human moderators to help combat spam but this does not fully deter spammers. Additionally, there are no federal or state laws (that i know of) against bombing someone else's website with irrelevant comments and threads in order to obtain a link for your own personal gain.
Other than moderating and software filtering, I wonder what other methods site owners use to combat link spam?