Complete a CAPTCHA before submitting an entry.
(Only necessary if the contest only processes one entry per contestant, and each contestant is limited to one account name.)
Hannes Naudé commented
I doubt whether this will be effective. This is effectively a challenge to those using autosubmitters to see if they can circumvent. Never a good idea to challenge a sufficiently smart person in this way.
As an example, it will be a simple matter to rig an auto-uploader so that the entire process is handled automatically, except for the CAPTCHA, which is displayed to the user, the correct answer is typed and passed along to the server. This somewhat restricts the rate of submissions, but for comparison purposes, consider that the average submission rate of Alan's end flood bot in the Sensor contest was one submission every 5.5 seconds. I'm pretty sure I can solve most CAPTCHAs at that rate or very close to it.
Far more effective would be to simply ban the use of sock puppet accounts explicitly. I might be naive, but I believe that contestants will abide by this. The contest community is built to a large extent on mutual respect and there is no surer way to lose that respect than to willfully break an explicit rule. Also this is a little easier to implement ;-).
This should definitely come to avoid automated uploads!! Regardsless of user login!!