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..
How come it is not up to him to tag that scammer for stealing his own work?
To me, that is a textbook example of a good reason for a negative feedfback and I just left one to Apt2022 as well. The only problem is that its a Newbie account in question so he will easily ditch this one and continue business as usual with another one.