You could set a cookie for each user, that way you could distinguish between different users on the same IP. Of course, a savvy user could figure that out and then delete their cookie to be able to resume downloading.

Anyway, whether you're tracking IP's or cookie's, each time a multimedia file is requested you user a server-side programming language to check your database to see if that IP or user has reached the limit. If not, serve the file and record the request in the database. If so, then show a "Bandwidth exceeded" message.

If you really want to block by IP, you can put commands in your .htaccess file. Search Google for .htaccess and "deny from".

There are server modules designed specifically to address this problem.

There is at least mod_cband, and probably others.

Thanks a lot! It's so useful and i'm gonna try to make it.