The reason why it might be a good idea for the admins to stomp on such behaviour isn't just that the behaviour is harmful in itself, it's to establish a culture of not doing that sort of thing.

The votes must all be logged already (or something functionally equivalent) because the system already knows to stop you upvoting or downvoting the same thing twice. Providing a UI to make it easy for admins to look for mass-downvoting would be the trickier thing, and indeed it might not be worth the effort. Though, on the whole, I think it probably would be, in order to establish LW as the sort of place where that kind of thing just doesn't happen.