In my own opinion, it is not up to you to tag him with a negative trust, what I think you should do is what you have done, and that is reporting him the bounty manager, is it up to the bounty manager to tag him with a negative trust, and if necessary, ban him from participating in future bounties..

So what I will advice is that, you wait for response from the bounty manager, he is the one to handle the case in the best way, you can tag the user later, but wait for bounty manager to tag him first plus take other necessary action on him.