BTW Whenever I have seen ppl camping irresponsibly in the wilds it hasn't been kids but ppl in the 35 + bracket. Probably myself included although I don't drink (much) and definitely not cans but bottles (preferably whiskey or wine or bottle conditioned beer or real ale in bottles).
If they are using tarps then prepare in advance by setting beer traps or other slug traps then librally sprinkle slugs on their faces. This works in tents too iff you leave the doors open. only trouble is the beer trap kills slugs so try to catch them by hand. If you catch the slugs alive then use any remnants of beer from the cans and dose the tents or tarps with it spo the live slugs will stay around. Not sure if it works but I like the dea of them thinking the area has a major slug infestation and that the slugs like humans.
I must admit that you guys are inventive. Do you use your powers for good too?! I like the one about putting stuff on the fires. Even if they don't burn fully they would still be unuseable.
If they leave their shoes or boots outside or in the porch and you can get them without being seen then a vindictive person could break any of their glass bottles (if they drink out of bottles and not beer cans) then sprinkle it around their tenst so they have to go through the broken shards to reach their shoes/boots that are just out of reach but visible from their tent porches. An alternative is to get the boots and shoes then tie them high up on a tree but put the broken glass andanything else sharp around that tree for some distance. If you can make the tree hard to climb too so they keep falling down on the sharp bits. Perhaps a pine but you break off the branches as you climb back down before adding the sharps. Please don't do this as I am only joking.
I would honestly hope I'd call the police. I do know a rural copper and unfortunately they cover a large area, much larger than a town bobby. It is quite possible to get many nuisance areas in their patch and policing it for this anti-social activity is not really viable. Although they won't know about it if you don't call them. Realistically the night bobby is probably in a car and is probably sorting out some road based problem at night. Either that or preparing for a night-tiume raid up a farm track. Seriously you have to be up very early to catch dodgy farmers. Realistically you won't change them and you won't stop them. Tough! You've lost your secret place. If you can't beat them, join them. Perhaps if you get a lot of mates round and make more trouble than them. I reckon you could get a few dodgy looking bushcrafters together round a large fire for a few weekends drinking a lot telling tall tales into the wee hours. Anyone up for that??