This is easier said than done. Spammers would just post bitcointalk dot org instead of a actual link and search engines would still pick up the text of the link. The underlying reason why these spammers post these links isnt to get forum users to click on them, it is to get search engines to recognize that bitcointalk has the link displayed on one (many) of its pages which helps the search engine rankings for the link.
A better solution would be to configure the robots.txt to somehow get search engines to not pick up posts by newbies that are not at least xx hours old. This would give mods time to remove the posts before search engines can pick up the links, and reduce the effectiveness of this spamming strategy to near zero.