Rose bushes. You absolutely need rose bushes. Plant at both ends and have several spots in the middle where they fill the space between the walls. One or two bushes will not do. You need at least four spots blocked by the rose bushes so that the homeless would need to push through to go down the path. Try to get the tallest ones you can find. For those who are reading this and wondering what I am talking about you must never have run into rose bush thorns. They hurt.
Madagascar Palm
Ouch!