Hows about you and your neighbours writing to your local mp to get gates put up at the entrace to the alley ways stating that cars are always being dumped in the alleyways . It worked around our area when scum where always robbing garden sheds . The council installed 10 ft high gates that wwent accross the width of the alleyway so you can still get a car down them . I,d also use carpet gripper on the top and back edge of the wall 3 rows right next to each other should do it .