It is something that could be looked at, however keep in mind you can see who turned in the bounty so if you saw someone abusing it you can just turn around and put a kill on site bounty on the people abusing it.

I might add an option for that

Yeah. You need to add option for this.

Before i saw this thread, my players already discussing their own tribemate/allied to kill and collect bounty.

Potential Exploit there

It is now an option, thread closed