You can't centrally firewall machines within the same subnet! Interfaces within the same subnet communicate directly with each other. They only send traffic to the gateway when the destination address can't be routed directly to one of their local subnets. You would first need to logically isolate those machines so they cannot route to each other. Then, you would need to do central routing (and firewalling) for them.
A hack, and it is a real dirty hack, would be to define every machine as its own subnet on the same physical segment and then define one interface on pfSense for each of the machines on the segment, then set up your rules. This is a really bad idea. It will probably break more than it fixes since the machines can't broadcast to each other any more and pfSense has to route every single packet. And even if you did that, since you'd be on the same physical segment, any user could get around it by just defining an IP in the segment they wanted to talk to.
The short answer it it can't be done.
-Ted-