Blocking a second router/dhcp source in the network

Hey all,

I am about to manage a network where users like to plug their own routers into the network, there is nothing I can do to stop it physically happening so i'm looking for a software solution. Each user pays for a cable connection.

What frequently happens is that they plug in the lan side of their router instead of the wan side, which breaks my network and is a big pain in the ass.

How do I block or override a second dhcp server on my network? What routers support a feature to deal with this?