It is not possible to set an event site to draft once it has been published, but you can hide it behind a password. Hiding an event behind a password means that only people with the password can access the event site.
Hide event site with password
To hide your event by setting a password, follow these steps:
Go to the event
Click on `Settings`
Click on `Setup`
Check the box `Password` under Who can see this event
Choose and enter a password under Password
Click on `Save`. Any visitor to the address of the event site will now be asked to enter the password in order to access the event site.
When you are ready to show your event site without a password again, check the box `Everyone` or `Invitation only` under Who can see this event, depending on who should be able to access the event site.