An apartment complex I'm working with provides internet service for all its
tenants.  A simple DLINK router is configured to a T1 service providing DHCP
service for anyone connected throughout the building making internet service
available.  The problem is that any user with a router having DHCP SERVER
enabled who plugs into our LAN on their LAN side will screw up Address
Leasing from our server preventing homeowners from internet access.  What
would be a better firewall hardware to use, and what services /
configuration can prevent this problem for the future?  Can MAC address
connectivity, or some other configuration other than static IP assignments
give my server priority over DHCP Leasing?

Thanks,

Dan