While it's true that we  shouldn't tag members for bounty abuse without very strong evidence of connections (which hasn't be provided so far), you as bounty manager still have right to remove someone from bounty campaign at your own discretion, right?

While connection is not strong enough for a negative feedback, i would personally kick that group of the accounts out of campaign as it's pretty much obvious what is going on there. Sooner or later they will slip, most likely by sending tokens to the same address or something along those lines.