I'm not published, and I don't have an agent. But this type of thing makes me sick inside, so I'm going to be very opinionated and state it clearly: if someone has done something to hurt/annoy/bother/frustrate/etc you, it is NOT okay to turn around and ream them online. Period. It's immature and unprofessional (and no, it doesn't matter how immature and unprofessional the other person is).
The good news is, none of the people (including the few agents) I follow on Twitter participated in this at all. The bad news is that I guess a bunch of people not only participated, but apparently wrote encouraging comments to the agent who attempted to blackball this person online.
You know, if the writer in question is really that difficult, let him dig his own grave. All that was accomplished through this was the revelation of the petty side (which we all have, but hopefully we're all learning how to keep under wraps) of this agent. (Um, I'm not afraid to name names, but I don't happen to know any names -- which is good, because I don't want to be perpetuating this mess.)
Have I ever tried to use my small online presence to make a difference when I was upset about someone? Yes -- when an internet company screwed me over (and I found out they had done so to thousands of other customers), I didn't hesitate to 'out' them online. But they took money and time from me; they didn't simply write insults to me.