the only way this would be feasible with vanilla iptables is if all those videos were streamed from some fixed port/range (or some other characteristic you could match)... i just did a quick netstat while watching a video on youtube and it seemed to me it was getting streamed via port 80 (HTTP) on the server... =/
the optimal way to filter this is with a proxy server... you could set it to, for example, deny access to any URLs that contain flash video/animation extensions (.swf, etc.)... in
squid (the most popular proxy server for linux) this is easily done using ACLs (look into the
urlpath_regex one)...
http://www.visolve.com/squid/squid24...ntrols.php#acl
