The only solution I can determine is making it more 'peer-to-peer'. Someone advertises their services as a bounty hunter, then you explicitly pay THEM the money to destroy the opponent (maybe with a deposit system to avoid scammers just taking your money and running). This avoids bias, as you could potentially hire any bounty hunter to take out any criminal, but it's still easily exploitable (friend 1 becomes a hunter, friend 2 is the one who gets a bounty put on him, friend 2 flies a sidewinder or something, friend 3 hires friend 1, friend 1 gets the money). It just adds an additional accomplice to credit-transfers, but at least takes out the possibility of 'friend 1 gets a bounty, gives the bounty money to friend 2, meaning that all the person giving the bounty did was give money to the gankers they dislike so much'.
TL;DR - there's a reason we let the government handle crime and punishment nowadays.
What is with the mentioned NPC solution?
By the way, you can also set reward in real life which manages to get the culprit. Or other example you can pay legally people to get your money from a person who didn't pay the bill (of course without violence). And there is the legal system where you can go to court (you need your own money) and can get what you want (when you win). Yes of course in real life there are also loopholes but technically if you have more money it is way more possible that you get what you want even in our democratic systems. Money rules the world even in real life.