Can’t park in my own space

Our parking area is secure, with the unit number clearly marked on each space.  We don't own a vehicle, so it is great to be able to offer our visitors a convenient, safe place to park when they come for dinner or to stay overnight.

Frequently however when I try to give a visitor our space I discover that it is already taken by another vehicle – I guess neighbours notice that our space is often vacant and that we won't know or care.

We have on-site after-hours security, but they seem unable to help – they just tell us to report it to the building manager (and in one case I caught one of the security guards using the space!  He is however no longer on duty in our building so I can't blame him any more.)

I've looked at various options to deter this behavior, but it is rarely the same vehicle offending twice.  Suggestions that I've come across vary from leaving notes on the vehicle (which I usually do) to dumping garbage over the windscreen, letting down tyres and other vandalism.  I don't want to resort to this for two reasons; (1) it's really not my style and (2) they know whose space they are using!

Best solution that I have come up with so far is to photograph the offending vehicle and tape a note inside the lift with the picture of the car, its number plates clear on the picture, stating that if this is your vehicle you are parked illegally and you need to move it now.  This often seems to have a quick result.

I'd love to have them towed or clamped, but I doubt this is legal (I live in Waterloo NSW, in the City of Sydney council area)

I have no way of knowing who owns the vehicles, rarely is it the same one twice, and I don't really feel like camping out in a smelly and damp car park to see who they are.  I'm actually a little surprised and annoyed at myself for letting this become such an issue with me!

Thanks for listening to my rant, any suggestions would be gratefully taken on board.