Maybe it's just me not knowing much about website design, but this seems like a problem which could be mitigated with automatic controls on the karma system. Like, for example, you have a limit of +/- n net Karma you can award to any given poster in an arbitrary time limit t. Or even that if your rate of downvoting any given poster cracks some ceiling it sends up an automatic mod flag that there might be an attack going on.

Ideally, of course, we could just abide by the honor system, but from a pragmatic perspective it might make more sense to set up stronger safeguards as an additional measure.