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.