Parking space isn't so much an issue where I work - we have several bike lockers and a few more racks, so unless there is construction and the bikes have to be moved, I don't think anyone has been fretting too much over abandoned ones...

Lockers in the locker room OTOH are in rather short supply. People do get evicted from there. Every once in a while an email notice goes out to confirm that you are using the locker or turn in the key. If you don't confirm, your stuff gets put in a plastic bag and put out....

One way I heard about to tell if someone is using a bike and just hasn't got back to you (if you've put up a notice) is to put a sticker over the keyhole of the lock. If the sticker is broken or removed you know someone has unlocked the bike (or at least paid some attention to it)