Terminal command to kick selected users off my network
Unless your desktop is the central proxy server you will have to go through the router. Just because you can see other machines with arp doesn't mean you can do anything with them.

Kicking them off the network via the router will usually be total - does the office staff depend on the network do do their jobs?

A better way is running your own DNS server. You can then simply redirect any time-sucking service somewhere else in a manner that makes it look like the site is temporarily down, like this:

*.facebook.com IN A 192.168.1.252


192.168.1.252 runs an Apache vhost site named facebook.com, it serves up a clone of the FB maintenance page for any URL.

It will take a very network-savvy user to figure it out, and you won't be the demon boss (at least not right away). Turn it on and off regularly to maintain the illusion that it's the other service.

Or for more fun: Upside-Down-Ternet.