You can set your business unit to only host free events. This will prevent an admin from creating an event that allows passes to be sold. To enabled this, navigate to your business unit dashboard and select the "General" menu item under "Event Settings".

From here you can enable "All events are free". When only free events are enabled, any event created within the business unit will be free of charge. All passes, including add-on passes, will be free. Please note that this will not override any existing events that have paid passes.