Tell them they cannot come back. It's in the rules.
Quote:If the enemy (character or characters) leave the game before two hours have passed, the player may return to the system. If the enemy then returns to the game they may request the player leave the system again until two hours have passed since the original time of death.
I read it that way:
You kill the pirate on your house navy ship in a move to protect e.g. mining in the system.
The pirate respawns (= "returns to the game") and you green message him: "Please do not come back to the system in the next 2 h".
Done. Zombie-Apocalypse averted.
That was easier than I thought... *burns all the Zombie Apocalypse Survival books*