Posted: 25/05/2014 at 17:30
I'd echo Fairygirl's suggestion of berberis as a thorny barrier. Quite a few years back at the side of my house children used the raised bed at the side of my house as a walkway, parent oblivious as they trampled plants and other wildflowers growing there..
Finally I planted berberis darwinii where they climbed the wall and more several feet apart along the bed. They solved the problem almost overnight. It didn't cost that much back then but it was worth every penny. It also stopped the litter they left behind. I keep it to knee height with a trim every autumn, and in between I have other wildlife friendly shrubs especially buddleja.
I can understand your frustration, people in my experience have little consideration for others ( there are exceptions of course), but thorny barriers are fantastic deterrents.