The L7 firewall rules most likely aren't working as a lot of Google services use QUIC by default - which L7 firewall rules won't pick up.
You'll need to use L3 firewall rules to block outbound UDP port 80 and 443.
That will force Youtube traffic to revert to HTTP/HTTPS which the L7 firewall rules should apply to and block.
Other than that, you can try blocking "*.youtube.com" in L3 firewall rules.