How do I change registration items from "Add to Cart" to "Closed"?

To close registration for all Registration Items for an event at once, click on Publish & Access and then Publish.

Here, under Event Registration, you will see a button that says "Stop Accepting Registrations". Click that button and all Registrations Items for your event will display as "Closed" to any people viewing your event site. This will also disable new registrations for your event.

If you want to reopen registration for your event and/or Registration Items, click "Start Accepting Registrations". This will enable people to register for your event and your Registration Items will no longer display as "Closed".