The hatcher that is selected and the vote (yes or no) are handled by the client software. The actual wallet holder has no control over this at all.
That's meaningless from a security mindset. If it's open source, people can change the client software, heck, even if it's closed source, people can change the client software with enough effort.
If you were to grab the source and modify it to always vote yes, then even if there were only 100 other voters in the system, you vote can only skew the vote by 1%, which isn't enough to cause damage and will only push up the vote threshold over time.
So now we're back to "what magical algorithm are you using to stop Sybil attacks?"