The default status of all units is 'Available' when not rented, and 'Occupied' when rented. Since there will likely be situations where you need to make a unit unavailable in Stora, here's how to do so:
Select the unit you wish to change from the unit list. Then select the '...', followed by Make unavailable.
The unit is now unavailable and will not appear as an option when you are allocating a unit to a customer. You can change the status of the unit back to 'Available' at any time by following the same steps.