Just report this guy to the bounty manager you are participating. You can simply solve it but this can not stop him of cheating because they are very expert of doing this specially in signature campaign so you need to be watchful.
But for scammers always and will remain, besides they can not be handled because the cryptocurrency market is not regulated by anyone, you just need to be alert and alert all the time, this is to reduce the scam that will happen in the future.