npnp
Couple of notes for others in the same situation:
OutSim and OutGauge ports are configured in the cfg.txt file, and if they are enabled they send packets whenever you are on track to any program listening to the port you have set.
InSim is started with a command, but InSim requires a handshake to establish the connection. The problem here was that the OutGauge was set to the same port as the InSim - the spam was from InSim receiving a packet from the OutGauge, and complaining that it wasn't the expected handshake packet.
The solution was to make sure that OutSim, OutGauge and InSim all use different port numbers - OutSim and OutGauge in the cfg.txt file, and InSim via command.
One other note: when altering cfg.txt, LFS must be closed. LFS rewrites the file with the current values when it closes, so any editing you do with LFS open will be undone when you restart LFS.