State-by-state management?

I have a firewall here that sees heavy use on a full-duplex 10Mbps fiber 
link. (Okay, so maybe not that heavy.)

On occasion, there is an existing connection which has matched an 
outgoing keep state rule which I want to specifically kill. However, as 
far as I can tell, there's no way to kill just a specific state in the 
state table. Thus, even though I might use:

ipf -f -
@1 block in on ex1 from 10.2.0.12 to any

... since there's state involved, the rule doesn't take effect except 
for *future* connections to/from that IP address. That leaves the 
current pig of a connection consuming all my bandwidth.

I find I have to take a sledgehammer to it and:

ipf -Fa -vf /etc/ipf.conf
ipnat -CFvf /etc/ipnat.conf

... and completely refresh all the rules, state, everything.

If I could instead operate by adding specific rules to impede the 
problematic connection, and then kill the offending states 
specifically, my life would be a lot easier.

Is there a way to do this? Perhaps by using some sneaky active/inactive 
rules lists or something?

Thanks,
Marc