I earlier commented that rather than looking to impose limits, we should be looking to incentivize behaviour we want to encourage.
The example I used was to increase curation rewards for each account upvoted in a day. This encourages curators to use a single account, and to broadly penetrate the market seeking accounts to upvote. I thought a 1% increase per each new account upvoted might work, but I've not done any math to examine whether this is a good figure, and I'm confident better heads than mine are better able to calculate an optimal increase.
Thanks!