Secure the physical network

Any thoughts on this one!

We have been asked to provide security to our public facing netwrok ports. Some of these are in building that have shared resources. i.e. other companies, partners, etc.

What we need is to be able to stop someone just plugging in their own laptop/PC and getting an IP address from the DHCP server.

I can not think of an easy way of doing this.

Anyone got any thoughts?