The easiest solution would be to block dns resolution to those sites in the internal dns server†. If your pcs go through a proxy, a better solution would be to block those urls in the proxy configuration.
Note however that although your manager thinks that facebook or youtube shall never be used in the office, you will find that it will sometimes be needed: someone managing a company profile in social networks, that video relevant to the company that you should see that happens to be hosted on youtube… and you will probably be making your users angry, so block with caution.
† they could overcome it by changing their dns settings or using proxies/VPN, but would server as a soft-block, and they supposedly aren't local administrators.
Update: Yes, you could list them in the
hosts file. But do you really want to add those entries on 120 PCs?