When you have the "Make a Reservation" or "Suggest an Event" template turned on in multiple calendars in your account, you'll see that there's an option to choose what calendars you book your event to. Any calendars that have "Make a Reservation" turned on will be listed in the "Choose One" drop-down list for your "Make a Reservation" form and the same applies to any calendars with the "Suggest an Event" template turned on. See the image below:

There's an option in the "Customize" menu for your calendar that you can turn on and the "Choose" menu will not show regardless of how many calendars have the same "Make a Reservation" or "Suggest an Event". This means that when visitors submit an event or reservation to your calendar, they will only be able to add that booking to the current calendar that they are viewing.

This feature is the opposite of the "Do Not List this calendar in Self-Book forms" feature. While that feature only removes one calendar from the "Choose" drop-down menu, this feature will not allow the "Choose" calendar menu to show at all.

To turn this feature on, follow the steps in the image below: