Yeah that's definitely a problem. it would need a bot filter of some kind. Many forums have implemented user hierarchies that affect voting power. This could work hand in hand with the admin hierarchy. All new users, including bots would have little voting power, and comparitively no influence to the vast majority of accounts that, in theory, would be used more often gaining more voting power... but this heavily depends on making sure human actions are rewarded, like commenting, and even implement machine learning on upvoting frequency etc, as oppose to simple things a bot would do.