I suppose you could put NOINDEX,NOFOLLOW attribute for ROBOTS meta tag and it will avoid Google indexing your pages at all. Read also related threads:
Why my miniBB forum is not searched friendly on Google?How do I avoid GOOGLE finding my board?There is no randomly changed content in miniBB and no kind of other tricks. It's just has a well-generated plain easy valid XHTML pages which are not the miracle to love by the indexing engine. Miracle? I believe not - just a good layout prepared for the content.
We are also not affiliated with Google anyway, however we love Google and Google loves us :-)