How can I set a time to deactivate my form at midnight?

I would like to have this be inactive at midnight tonight.

I think you want to simply set a timeframe which your form will be disabled?

Yes, this is possible. 

Please see http://www.jotform.com/help/137-How-to-Limit-Submissions-in-a-Form for further instructions.

You will need to set an Expiration Date and Time.