Trying to prevent the same vote by multiple people

I need help. I'm going to be running a contest for our company that let's people submit guesses as to when we will hit a particular goal. They will choose the DATE they think it will be and in a separate box they will choose the TIME. The closest person to the correct date and time will win. I can't figure out how to make sure that the same guess for DATE and TIME cannot be submitted by multiple people. Once you cast your vote, that date and time should be blacked out. How do I do this?

You are currently using our Appointment field. That should do it.

If you want that only one person can choose a date regardless of the multiple time selections, you have to set the "Maximum Appointments Per Day" to 1.