How can I make selections only be chosen once for a time slot?

You can limit the number of items/tickets/slots available on your form by using our widgets, such as our Inventory widget:

http://widgets.jotform.com/app/inventory

We also have other widgets that provide this capability, and these widgets are Ticket Purchases, Gift Registry and Appointment Slots:

http://widgets.jotform.com/widget/ticket_purchases

http://widgets.jotform.com/widget/gift_registry

https://widgets.jotform.com/widget/appointment_slots

You can take a look at the following guide on How to Add a Widget to your Form.