How to block some IPs with SuSEfirewall2 ?

Hi, I'd like to make a number of IPs unreachable from within our LAN. The PCs in the LAN have internet access via an SuSE 8.2 box that does NAT for them. Now I'm rather convinced that /etc/sysconfig/scripts/SuSEfirewall2-custom is the place to stop all packets to and from those IPs. I tried to set some rules but didn't get any result. How can I stop any talk between my host and certain IP adresses ? Thanx in advance Andreas