There are a number of addons for Chrome/Firefox etc that allow you to block sites, usually for staying focused during work hours.

If you want to do it network wide, you could utilise something like a PiHole, but that's a bit extreme for blocking access to one site.

The other option would be to put an entry in your hosts file for www.youtube.com etc and point them at 127.0.0.1.

Although doing that for just one domain is overkill.