How to limit reservations inside of my form?

When someone reserves their timeslot on the form - how do I show that the timeslot is no longer available. That timeslot has been booked?

If I understood you correctly you want to limit reservations inside of your form.  

With JotForm you can use the following widgets in order to achieve what you want. 

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

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

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

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

On this thread http://www.jotform.com/answers/519509 my colleague explained in detail how to set Gift Registry widget.

Hope this will help. Let us know if you need further assistance.