Easiest way to do this:
# give "Sentry mode" order
# issue "Move" order for that fleet to any other sector. They won't move
# try out anything else and it won't work either
# I had player 1 send a fleet to attack a sector that had player 2 in it. The fleet arrived but didn't move, it just fired a few missiles at the player if he got in front of the ships.
# I had player 1 issue a move order to that fleet yet the fleet didn't do that, still sitting still and occasionally attacking player 2 if he was in front of the ships.
# I had player 2 leave the sector and as soon as the fleet got unloaded, it started moving.
# The fleet got loaded again by player 2 while it was moving and I issued a new move order as player 1
# The fleet ignored that order and kept moving to its original move order sector