well, the bad news is that: long story short, you can't block youtube through firewall aliases (unless you got much time, patience and technical knowledge to enter every single IP youtube uses).see this link for reference: https://forum.opnsense.org/index.php?topic=5279.0.The good news is that there's a couple of solutions Depending : 1- Use DNS to block access to youtube which is simple enough to do, but users can use VPN to access blocked websites.2-Use IPS (Intrusion prevention system) deep packet inspection to block certain websites and VPN but it's hard and not fail-proof.3- Ideally use a transparent proxy server to filter traffic and block VPN for good (again this isn't for the faint of the heart).You should study your case and choose the appropriate solution.