Sorry to hear of your problems. I worked in security for a while when work was in short supply. We had similar problems at a number of sites and found putting things below the walls worked. Very spiny prunings were always welcome and we also had success with empty plastic bottles, old pots, anything that was difficult to drop down onto. Cages made from wire over the louvers would help, fitted proud of the louvers like an old fire guard. It may be possible to get ready made cages used for protecting people from touching external heating exhausts like the ones used for oil and gas boilers which would still allow the louvers to open.
