Filter in-progess session that has been NATd?

I have a box in router mode with public static IP address on the external interface. Say I have a host outside the country sending a UDP stream on port 5060, which NATs over to the phone system, and is reflected as so in Sessions. I go to Packet Filter and add a rule to REJECT coming from the external interface and that host address. But in sessions, and in bandwidth monitor, the session continues, bandwidth still fluctuates, etc. So I try turning off or deleting the port forward to the phone system, but the session still continues. Reboot the server or restart uvm and the session is gone. Is there a cleaner way to kill a session? Does packet filter use '--state new' on rules so in progress ones don't die? Or is the session somehow truly dead when I add a packet filter?

Filter used:
Action REJECT
Source Address: 178.211.44.67
Source Interface: External