If you wish to prevent anyone from selecting an ad spec which is already fully booked for that time frame follow these steps:
- Navigate to the Overview Tab
- Select Administration Settings
- Click on the Tab Administration Settings
- Check the box: "Disable the selection of fully booked cells"
- Hit Save
If you open the booking calendar now, the users can't select a booked item/cell and add this to their basket anymore.
Make sure you have no exceptions to the rule because this option blocks the selection of booked cells always!
Alternatively you might use the 18.104.22.168 Exclusivity Check to prevent Overbooking manually.