[MetalMilitia]

By the sounds of things there isn't much you can do. If your landlord isn't interested and the tenants themselves fail to listen to reason or employ any basic parenting skills you're kind of up shit creek.

All you can really do is talk to them and see if they can at least sort out some of the worst problems - like your mail being stolen. If you lower yourself to their level and start trying to piss them off back, they'll most probably use their extensive experience in being asshole neighbours and make your life much worse.