As someone who's been subjected to several of these rampages under a series of screen names, I think the best way to react is to act pissed off and create several threads complaining about Gagh negging you, making impotent threats and absurd demands. In QSF, even.
This tends to extend the rampage, which is kind of funny. And missmanners might pity you and give you back your karma, which is the one thing that seems to sort of piss off gagh.
It should be pretty clear that you're not personally upset and don't attribute real animosity to the negging, though. Because it's meaningless.