Hey Sanamarco,
Hoops right, theft is a big problem here in WA. Depending upon where you kite, just about everyone I know has had their cars broken into and kites, wallets, phones, laptop (mine), boards and just about anything else of value has been nicked.
Worst beaches are Woodmans Point, Leighton Beach, Contacio's Beach, Pinnaroo Point and Mullaloo Beach. These are all the popular kite beaches and I suspect if you park your car at any of the lesser used beaches along the coast you will get the same treatment.
I do not put kiting stickers on my car because if you are the only one out flying a kite on the water and your car is the only one in the carpark with kiting stickers all over the windows ..... well it dont take a genius.
Drive an old ****ter, leave nothing in it and the windows down, take all your gear to the beach with you and DO NOT LEAVE YOUR CAR KEYS in the kite bag. Many kitebags have been nicked even when empty but still containing car keys.
Good winds and slow painful death to all theives,

West Oz is totally awesome for a windy summer, pity about the carpark vermin