From the planning you can manually disable the reservation of classes. This way web users and receptionists cannot book people in. Please note that this can be done only by users with the appropriate rights.
In order to do so, open the Calendar and select the slot of the class you intend to disable booking for.
Once you have clicked on the arrow near "Save Changes" a new button will appear. Click on Block Bookings
The class will be marked with a red symbol
To enable again bookings you will have to repeat the same procedure.
(Please note that in each booking window the user will be informed about the unavailability of bookings for that specific class e.g. Home Page - Timeline)