Signed-in users can also find this guide, with screenshots, under Help in the app.
A blacklisted vehicle shows Blacklisted vehicle in dark red on every plate check, and the building will not accept guest bookings for it.
Open Vehicles in the menu.
Find the car. Type the plate in Plate, or set Status to Blacklisted to see cars that are already blocked.
Press Blacklist on the vehicle's row. The status changes to blacklisted right away.
If the car is not in the list yet, press Add vehicle, choose Visitor (saved for next time) as the Kind, enter the plate and press Save. Then press Blacklist on the new row.
To lift the block, press Unblock on the row.