The event has taken place and you would like to keep it accessible for your paid members. Alternatively, you do not want to sell any more tickets. Or maybe you are taking registrations on a third party platform and will be importing your attendees. You would need to use the option to close your event for registrations.
You can find this option under Event Setup > Settings > Checkout experience > Registrations Open?
Once you've closed your event for registrations, this will remove all CTA buttons from your event. It will also remove the Book my place from your header links. Users will be able to navigate your event, but users that didn't already have a ticket won't be able to access your content.
Please note that if you are importing your attendees, you have to open the event for registrations while you import, then you can switch it off again.
If an event is closed for registrations, then you'll also gain an event back on your event quota and your attendee tally will also go down.