The Forum plugin could have an enhancement to check for URLs as to whether they are broken or not. My suggestions is this..

• Each time a user enters a particular discussion page, the forum plugin scans all the replies for URLs in the various posts.
• Does a head request to get the status code of the URL.
• If it is 404, then add an entry to a new model Forum URLs or something in that effect to log broken URLs. The forum moderator may choose to either remove the post / remove the post or whatever.

This would be a good feature mainly because URLs tend to break sometimes as domains expire or resources are moved and it's weird for forums to have many broken links.