DHCP and Netware

On a network running Novel 5.1 – they were obtaining IP addresses from the server. We needed to install a Firewall – due to a direct cable internet connection. The firewall was setup to enable DHCP – after awhile the workstations were pulling the IP addresses from the server and not the firewall. It was necessary to configure each machine for addresses. How can I get the server to stop trying to assign IP info. to allow the firewall to do that???