The mission of the internet app developers over the past years has been to prevent network administrators from fiddling with their applications.
Put everything in https, make DNS encrypted, add additional measures to https to make filtering even more difficult (encrypted SNI), make it more difficult to do state tracking by using UDP (QUIC), move everything to "content delivery" services that you cannot block because you would block more than one app or site, introduce services like "login using twitter" or "login using facebook" that make it impossible to block those services because you would block their login facility as well, etc etc.
So now your position has been reduced to a facilitator of network traffic. Influencing what your users are doing is made impossible for you.
Anyway, there is no reason to block Youtube. If you want a datacap, make that. Do not try blocking sites because they cause too much traffic for you, their role will be replaced by other sites that do the same.