Just go to PiHole's "Domains" page, in the box labelled Domain, type youtube.com, enable the checkbox for Add domain as wildcard, then click the button labelled Add to denied domains.
Now youtube.com and all of its subdomains are blocked, for all clients.
If you wish for it to only be blocked for some clients, then assign your clients to groups, and set the setting appropriately on the domains page.
