You have to force your management agent to act here. You may need to contact the directors to get the agent to take action. You could suggest they issue a letter to the owner saying that if the car is not moved by the end of the week, they will be forced to have it towed. They may want to take legal advice on the correct wording and add something like "if you do not respond to this letter by removing the vehicle, we understand you are happy for us to have it towed". At the same time, get the clampers to put one of those hard to remove stickers on the window saying it will be towed if not moved by the end of the week. Also, make sure that the apartment owner responsible has to pay the towing fees by adding them on to the account for his apartment.
On a side note, be completely sure you are using the right space. Someone in my building complained about a similar issue and then discovered she'd been parking in the wrong space all along - hadn't read her lease properly.