The bounty have a manager and team project right? then they should solve it the best way to prevent the cheater for their own project. There is no guarantee at all a bounty without cheating, always has some cheater. But if the project hiring a good manager, at least the manager knows how to make the cheater not too much by limiting each application by using email or device also using POA to check and verify all entries.
If you want a bounty 100% without cheater, is impossible.