What people fail to realise in spamming forums in order to create backlinks to their sites is that if the site that is linking to you is totally off topic to your site, google will not give that link any credit, in fact it will likely degrade the target site. For example, if your site is about dogs, and you spam a forum about cars with a link to your site, google is not stupid and will know that the two are not the same thing, and so the dog site will be penalised and not rank so well. With google's Panda update last year, it has cleared out many duff links and de indexed many pages and sites from its database. JC Penny in the States (massive department store company) used low value back links to its site, when the update from google came along, it wiped everything, and the next day JC Penny's web site was nowhere to be found on google. The marketing department had to start again, but this time, the links they got were top quality to prevent that from happening again. Google is a lot cleverer than people think and these spammers are not only wasting other people's time in having to delete their posts, but they're wasting their own time too.