How to make a selection unavaible

As my vendors register for our event - many vendor & product categories fill up.

How do I reflect these are closed & no longer available.

You can achieve this when using the Inventory Widget.

Here is a demo form https://form.jotform.com/61471146729964

As you can see in the demo form, every time an item/products was used, the inventory count diminished. 

You can setup your Registration form with the same concept.

Set a limit to the number of registration available. Once all registration used up, the user will not be able to register or used the specific item.

Thanks.