Bougainvillea or climbing roses to form a barrier? Hard to say without knowing what the space is like. In my area, every time someone builds an extension there are these massive rocks dug up that just get carted away in a tipper. I'd probably ask them to dump a few in the way so people couldn't pass through easily. If you can afford a motion activated sprinkler that would be funny to watch and has the element of plausible deniability, but I think some of them are actually marketed as bird deterrents. Could work for people though, depending on the range and where you point it🤔