Several (flawed)things, limit voting to ip, mail confirmation, but I think a captcha would be the best trade-off between inconvenience for the user and blocking bots.
Captcha makes it harder, but it doesn't block bots. You can use Python's sphinx library (the voice one) to turn those mp3s back into the original text, then use mechanize or windmill or whatever to re-enter it.
Choosing a new salt that isn't easily obtainable in a flash applet would certainly help :)