Couldn't you find another way to limit the bots? Off the top of my head:
- give everybody 10 votes/day
- have yet another bot posting three images, once, twice, or three times a day: one pretty picture, one fortune cookie, one captcha ....
- replying with the captcha gets you more votes
Yes, there are many ways to fight the bots. Limiting everyone is not it. If anything now we'll have a greater swarm of bots.