I was in a similar situation, but I fought fire with napalm. My new neighbor got one of those shitty hi-fi systems with a sub apparently and separating us was only a thin wall. Our shared landlord and authorities were both powerless to fix the problem, or just didn't care enough, so I took it in my own hands. Unfortunately to my ignorant new neighbor, there's always a bigger speaker and it just so happens that I have a touring grade PA set - I am talking tops and subs with 130+ db output power each. I placed my speakers facing our shared wall and whenever he would crank up his hi-fi, I'd put on noise cancelling headphones and blast him right back at about 20-30% volume of my system which effectively turned the wall on his side into a giant speaker. He persisted for about a week and then gave up. Then tried it again a couple of weeks later, only to quit for good. Giving them the taste of their own medicine is most effective.