Their are a couple of different options to stop people from registering for your event.
Under the Event Settings tab in Manage Event, you can turn off "accept online registrations" when you want to prevent anyone from registering.
If you want to allow only certain people to register, you can password protect your site in the Event section.
If you want to stop registration for a specific registration package, such as a foursome, go into the Manage Event tab and select Registration Packages. Click on the package you want to change. From there you set the package as "Inactive".