Firewall/NAT

Heres my delima. I have a router that serves as a L7 QoS, Web Cache, DHCP (where i can assign/block addresses by MAC + IP address) and has a heck of an assortment of monitoring tools. one problem. the firewall SUCKS! according to the techs theres no way to allow just the rules i want and block everything else. maybe im off with how m0n0wall does it but i think m0n0 does this quiet well. long story short. i want to use my m0n0wall with its 3 NICs to be a firewall... thats it. no routing no dhcp... just a firewall. is this possible? if yes how? I know this has probably been discussed, i just cant find the answer im looking for. THANKS IN ADVANCE!

If you are using all 3 nics then it will be a router because it will have to route between it's 3 subnets.  It will probably do what you require, I think that a diagram of what you are trying to achieve would be useful though as I am struggling to picture what you are trying to achieve and where the monowall would sit on your network.