Is there anything that keeps sock puppets from voting?

The limit on total downvotes proportional to karma gives you more than you'll ever need unless you're planning to downvote the world, but it does make it significantly harder to manage a sockpuppet army.

You could potentially use sockpuppets to vote more than once on someone's posts, if you feel so inclined, but all your socks would individually have to be productive contributors in good standing, and you're limited by your total contributions in the same way. If we're talking hundreds of total downvotes, pushing socks' individual contributions into undetectable territory would entail tedious account management and some pretty serious compromises in terms of status on your main account. I can think of a couple ways of finessing this with automated help, but they're pretty fragile and easily detected.