I have to agree it is a rather big problem, but the one that is manageable if you know what you are doing. There are few tactics that have helped us manage spam-posts just to build up backlinks.

First, we say it up front in the rules during the registration that our forum is a nofollow forum. At least those spammers who actually take a second to glance at the rules get that information.

Second, we proactively monitor all posts. And I mean every single post. While it will take a lot to get spammers' posts removed physically (it has to be pure garbage), if we get the feeling the member is pursuing a "post and run" strategy to get their backlinks up, we simply delete their signature. Most commonly, those signatures have little to do with the content of the board, so they do more harm than good anyway.

Third, we use other tactics to make sure spammers do not even get the chance to post in the first place (a technique I discovered by accident). Those I won't talk about in public as I do not want spammers to learn them and adopt a work-around.