Hi Vitaly,
HTTPS won’t work since NBAR can’t look into the packets. I don’t think Youtube publishes a list of all IP addresses that they use, maybe you can lookup their AS number, find the IP addresses and block those:
https://www.ultratools.com/tools/asnInfo
If you enter “Youtube” you can see that they use AS36561 and AS43515. You can lookup those IP addresses and block those.
Perhaps a better method would be to fix this using DNS. Use your DNS server so resolves youtube.com to a custom webpage and configure your firewall so users can’t use another DNS server.
Rene