exclude or blacklist a none joined computer from local network

Hello

Not sure if this is the right forum, but here goes.

I have a users that brings his own computer at work, and he keeps pluging in the network cable in to his computers - and there by get access to our local LAN. 

He has been told that he cannot do that, and need to access the gues wifi network if he would like to use his personal computer.

How can I block his computer from our domain and that he cannot use his personal computer? 

Is there an easy way? 

I have his mac address though :) 

Regards

K