Eat loads of eggs and nail the boxed to the wall. Even better, wear an egg box on each ear, so you won’t hear anything, ever. Oh, and buy a drum kit to play whenever they use the shower, they’ll think their new shower is knackered and start only using the bath.
You would have to have the egg boxes within the room to kill the noise
To kill noise from above flat we had to cover up the entire ceiling with Two 10mm pieces of rubber on top
of each other and then Two boards of plaster board to kill any noise from the above flat.
This was recommended by an Architect to pass the flat conversion.