event: map initialization condition: none action: Player - Turn Bounty on for "playername"
just type your human players in,for example u made a map with 4on4 and 1 computer on each side, so can make: turn bouny on for player 1-3 (4 is the computer) and "player 5-7 (8 is again a computer)
so you will get now only bounty for killing a human player unit