Wouldn't a simple captcha get rid of the bots?
Since the voting takes place at the blockchain API level, this would not work. There were a lot of discussions about it when the platform was starting to take off, and the general consensus was that there were going to be people who used bots whether we want them to or not.
With this in mind, the best approach is to find a way that bots and humans can at least 'get along' and even potentially complement/benefit each other.
The overall response from @biophil is totally correct though. Good curators (ones that want to make money) need to find ways to innovate and detect quality undiscovered content before others do. With the 30 minute limit in place, it forces them to look for things that are not already 'popular'.