I've marked those in our database, as inactive.
However, please bear in mind that your service, just like the likes of Rapidshare etc, are a haven for the bad guys to share and spread malicious files - it is this reason that people block your site. If you started being alot more pro-active, and actually checked the files users upload, for malicious content (how you do that is up to you), you'd find people would be less inclined to block your site.