Are you sure you know what you're doing?
I don't know the amount of visibility that the search engines (considering that they show up as users without profiles, I assume they're preregistered by the forum and therefore may see signatures anyway) have. This move is effective in a "I don't vouch for this link, make the link points 'disappear in transit'." kind of way. Nofollow is honored (except by Yahoo I hear) but the association, especially if the link anchor text is the url and notably if the word combinations between pages are unique enough, will be there.
If there's a tracker on either page (or the browser), and then if there's a statistically significant amount of traffic through a link, a search engine may give more weight to users and award points anyway in regards to potential webmaster error or "points shaping attempts".
I would say in the end this move could be called a partial solution to the problem.
A better solution would be to prune inactive forum users, or their profiles at least. It solves both DB size issues and "
Silent spammers", which do not even post.
Of course the best solution to the problem would be to get your hands dirty and look at links yourself.