Hello,
Thank you for posting on our forum.
What you can do is make the booking form visible only to registered users for certain events, and make it visible to all for other events. To do this, from the Event Edit/Creation page, enable this option:
This way, only users that are logged in can view the booking form and thus book the event.
You can also further customize this globally by limiting booking to specific user roles.
Please read this section: https://webnus.net/dox/modern-events-calendar/booking-settings/#6-_Who_can_book
Marc