Why don't you pull out the contract you signed and see if it says anthing about noise after certain hours. These people could be breaking the rules of the lease and the landlord could step in and straighten it out...or they might lose their flat. Your neighbors know you can't do anything to them but they also know the landlord can.
If that doesn't solve the problem, I'd get music you love and put it on very loud to block out their noise. Then guess who will be knocking on YOUR door! Then you just might be able to reason with them.