I don't think you all have mentioned one limiting factor yet. You can only vote once for each node. This means that for each login trying to attack another, you can only -- the amount of nodes the victim has.

Which means that lower level persons would probably not have the bazillion nodes (chromatic has 652ish) needed to do serious harm.

Or in otherwords, the amount of affect the attacker can have is based on the verbosity of the victim. So lower levels get affected less and higher levels get affected more (generally), and since raw XP means less at higher levels, it would probably balance out. (More accurately, your level is not the bases, but rather the number of nodes, but generaly more nodes means higher level).

So anyway, my take is that it should be discouraged socially, but no action need really be taken (if a truly malicious attack was made by someone with multiple accounts, (regardless of the fact that would take a good amount of effort), something like this would be easy to spot and simply remove manually).

If you must do something the idea of limiting consequetive -- votes on a person sounds reasonably simple and not too restrictive. (A less restrictive but harder to code example would be to limit X percent (such as 10 or 20) of your votes per person per day, -- or ++, at your option).

Enjoy,
Gryn