Me and my ex had this problem too and ours was in an underground car parking with fey fob entry only!
Apparently people who used to live there would let themselves into the building and open the garage shutters from the inside as they kept the same code for the main door. It would get especially bad around Christmas as it was a useful city centre spot.
We had signs up about clamping but they never did any good and even when we'd try and ring the clampers they would never come out.
In the end people started bying these mini bollards which lock up and down. Bit of a faff but stops people stealing your space.
http://www.barriersdirect.co.uk/categories/1-parking-posts?gclid=CIr0k73vsJ0CFdRb4wodaQyQkg