Putting notices on cars parked in visitor parking spaces

Our EC places stick-on notices on vehicles which park in the visitor parking spaces for their own personal use.  It is obvious when viewing CCTV that the drivers of the offending vehicles are NOT visiting.  They follow cars into the car park, park their car and then leave the premises.

Our block is conveniently located to the Sydney CBD and some of the local businesses get wind that there is available parking in our lot.

Question:  Is if illegal to put a notice (stick-on) on the windscreen of the offending vehicle?  Or should the notice be placed on one of the side windows?