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 3 dots followed by Make unavailable.
The unit is now unavailable and will not appear as an option when you allocate a unit to a customer. However, you can change the status of the unit back to 'Available' at any time by following the same steps.