Prevent multiple form submissions

[nbarnum]

Right now if our page loads slowly then customers can spam the submit button for the contact form.  What do I need to add to prevent the customer from doing this?  It can't just be a simple onclick function because if a customer forgets to submit a required item the button becomes unclickable.

 

One way I think is the button should be disabled until all required fields are filled out and then disabled after they click submit.  The other is that if there's no validation issue then the button becomes disabled.

 

How do I do this?

 

Version 1.6.1.4