It used to be if you blocked port 6672 inbound/outbound to anyone you didn't want to connect it would set you up in a solo session. This seems to kill sessions in RDO though, so I'm guessing Rockstar has some sort of pulse to check if you're reachable or not, and if it appears not, the game disconnects you entirely. Putting on a wire-shark to monitor which IP's that port is accessing might allow us to identify where they're sending/receiving the pulse check from, and if you permit just Rockstar to connect, allow their pulse check but not any peer to peer joining.