I would happily implement this if I could remember how to get into the server/account details ;___;
This is a grey area between a bug and a feature request; youtube in sigs should be disallowed intuitively, but for now just post here anyone discourteous enough to use them.
There should be so few youtube tags used in signatures we can just tell the users individually? I don't see a huge need to enforce this (and don't know how easy it is to do so in phpbb without some modding).