multiple subnets

I have my untangle set up with 2 subnets with multiple Ethernet cards

one subnet is 192.168.1.0

the other is

9.8.1.0

What I want to do is separate the subnets so that they cannot talk to each other. Right now, I notice if I cannot to either one, I can talk to the other network, but I want complete subnet isolation

is this possible?