It still increases the effort required at very little cost to other users: an easy fix that should at least remove the low-level vote spamming.

People are always going to try and vote-spam: low effort=high payoff. The only way to fix it is to increase the effort (without discouraging others), or lowering the payoff.

The problem with giving users the power to lower payoffs is the flame-wars (or the current run of downvotes). Potentially a fix that is less likely to be abused is to simply cap the amount of the reward pool an individual can claim in a day/week/monthly period.