Volunteer Unique Task Sign Up

I would like to create a form that would allow only one person to sign up for each individual job, then the field is either uneditable or hidden so that duplicate sign ups cannot happen.

In the example below, John Snow has already filled out the form and chosen the 4:30 time slot.  I would now like for the "4:30 - 5:30 - Enter first and last name if you want to sign up." field to disappear or become uneditable so that we don't have two people signing up for the same slot.

Hope that makes sense... see below:

Hi,

How about if you do it like this form http://www.jotform.me/form/42542921171449

Using the Gift Registry Widget

Tell us if this solution cannot work for you.

Thanks!