What about limiting how many times per week/month 1 users votes count towards another?

For example, if I upvote your comment or post today then upvote your comment or post tomorrow, the vote tomorrow doesn't count. This could help prevent bots from voting for the same user over and over again.

Of course it does have the problem of "just make more accounts." Perhaps it could be IP based as well? It wouldn't completely solve the problem but it could help mitigate it.