Love Ahjno's idea of the alarm clock. How about setting several of them hidden away in the brush but close enough to disturb them then stagger the times that they go off through the night. Maybe leave a note on each of the clocks saying: "Take your crap home with you" repeating it as each alarm clock is discovered. The last clock can have the note:
"Do we have an understanding? If not, it'll get worse, much worse, I have friends on an internet forum who are just full of nasty tricks they suggest we play on you." Muhahahahahahahaha! (no point letting them now there's only one of you.)
Could work.
As for confronting them directly and appealing to their sense of fairplay and consideration for the environment. I thnk their actions have shown that they have niether. Don't waste your breath or risk a kicking, cos' it could happen. Confrontations are best left to the professionals who can deal with it, unless you're a bouncer or Mr Intimidation himself then be my guest and have a word with them.