If i happened to be bounty manager, the only solution to stop those fraudster is to ask all participants to link their media details to the Bitcointalk forum profile, by this none will complain about using their details to submit for a work. In addition, all participants should filled their wallet on the btt profile as well too.
It is not that easy to avoid or to minimize the number of cheaters. Cheaters gonna be cheaters and they will have many ways to cheat. It is proven for years already where there are many people get caught cheating in bounties with multi accounts.
Cheaters will always ready with any conditions from the bounties. In case of linking the social media accounts with the bitcointalk forum profile, cheaters will create more social media accounts and bitcointalk accounts (even buy it for cheap). This way, they are still able to get more tokens than normal user with 1 account only.
If you are to be bounty manager, how will you deal with such situation while it is hard to detect that those multi accounts are owned by 1 single user?