Noton - 12:58 pm on Apr 23, 2009 (gmt 0)

Its hard, as while you can do some things to stop it showing up in link:www.mydomain.com searches, for example: making it a non-clickable link, just text version of the site. While adding rel=nofollow will stop Google, it will not stop Yahoo from listing the site in link:www.mydomain.com search.

Now both those things fail on users who just search for your site, as since the text is on the page it gets indexed and could show up in the results for search of your domain.

I think your best bet is a combo of the above and your last idea, use a dummy domain on the profile, in text, this way no links are indexed and you real domain is never indexed along with the pages of the forum.