To hide registration buttons on your event page, all you need to do is update your Registration settings on your Event's Dashboard.
Go to 'Registration/RSVP > Setup' in the left-hand panel.
Scroll down to the "Configuration" section and uncheck the checkbox next to "Registration is Active."
Once unselected, click "Update."
When you go back to your event page, you should see the "Register" button removed from the top navigation and the event page itself.
If you want to re-enable the button and make it active, just select the checkbox and update the page again.