Ah ok.. I guess the easies way is to tamper with the group.cpp file and make sure you cant group at all ....
But its going to be realy hard to do any instances at all if you cant get healed. But it might be fun too hehe... Constant backstabs...
But if you make them unable to group then they wont be able to enter same instance anyway... So what you want is healing and damage(?), aedamage to be FFA even in a group right?
Should be some flag type isFriendly in combat or similar for groupmembers or so... But all those things will be kinda hard to do since if you do it in town or similar alot of NPC:s will get annoyed...
But then again just ignore isFriendly just for PC:s and it would work somewhat what you want....
Look in combathandling or similar
Good luck
Bjerlk