The best one I know of is
Event: (player owned unit event) a unit ownec by (player #) is attacked
Condition:
Player comparison- owner of attacking unit is equal to (player#) <----an ally player
Action:
Unit- remove/kill (attacking unit)
that way, anyone who tried to BS looses their unit that they try to BS with. But I'm sure you can see the problem with this trigger. It runs and checks every time a unit is attacked, and with so much attacking going on like there is in usual footy wars maps, it can cause some a bit of lag in late games.
Thats just triggering. How I would do it would not involve triggers. I would have each group of players (assuming its 3v3v3v3, correct?) have their own armor and attack type (i.e. normal, seige, medium, unarmored, ect) and change the properties of the armor and attack types so that players do normal damage to the armor types that their enemies use, but absolutely nothing to those of their allied players. For example, one set of 3 players has medium armor and magic attack. Magic does normal damage to all other armor types, but does 1% damage to medium armor (the armor type of that player and his allies). Its a bit complicated, but its by far the most effective.
-VGsatomi