From looking at these bots on steemd, it seems like they were all made on the web UI via a fb or reddit account.
That is to say, there wasn't an actual investment, just the 3 steem you get for free.
A simple way to prevent this would be to simply prevent voting at all until you have something like 5x the number of steem your account was created with. 15 steem is small enough so that its easy to buy, or to get with even an OK post, but would probably discourage a bad actor from mass producing accounts.