This might surprise you to know but there is no way to control who participates in a movement. GG has made clear from square one that this kind of behavior is totally unacceptable. If someone goes ahead and acts like a fool anyways, what exactly do you want everyone else to do?
The best that can be done is banning such people from participating in the discussion forums, but this doesn't control whatever they decide to do on their own.
You're mistaken if you think that decentralized movements cannot be trivially co-opted. By their very nature, they are controlled by those who speak the loudest and command the largest audience.
So I must ask, who on your side of the #GamerGate hashtag speaks louder and and commands more followers than Adam Baldwin?
If you do not possess the social capital to overpower the voices of unabashed harassers, then your continued association with the hashtag only damages your cause.
(Note that I speak as someone who once worked in games "journalism" and is completely fed up with how shitty it is (not that the games industry at large is really giving me much to hope for).)
Maybe it would be wise to stop participating in the movement if everyone outside the movement seems to agree it's about harassment and misogyny? It doesn't look like GamerGate could achieve anything useful at this point.
The best that can be done is banning such people from participating in the discussion forums, but this doesn't control whatever they decide to do on their own.
Again - this is analogous to the vandals at OWS.