Same situation here, rented a nice apartment in converted old townhouse.
Downstair neighbour was a working man, didn't hear much from him - then he moved out, 2 girls moved in and Wed, Thurs, Fri, Sat about 2-3am they would crash home with randoms, all making a right F*cking racket downstairs - music, shouting, squealing laughter.
Me and my mrs worked hard, she worked saturdays and sundays too - so it was horrible for her not sleeping Fri and Sat nights then going to work. It made us miserable - so I had a quiet word after about two weeks of this, saying we've had enough, that we can even her what they are saying and I also mentioned it the land lord that I had a polite word with them.
No sooner did Thursday night come, the noise started again, Friday night was quiet - and then Saturday night had some sort of riot down there.
That Sunday morning, about 8am I nipped out in the car to my mates house, borrow a bass bin and his 400W slave amp - and I set up a Rennie Pilgrim DJ on loop from my CD player.
Some of the tracks he spins have basslines that literally you cannot hear but can see the furniture vibrating. I faced the bassbin down to the floor right above their beds - and I fired it up. The whole house shook - and then me and my mrs went to my mothers for Sunday lunch and I got back about 5 o'clock.
They'd rang the landlord at midday after going nuts - whom came into our flat, laughed to himself and switched it off. He then went down stairs and ordered them to pack their stuff and be gone by the morning.
We'd been there 3 years with no issues and they'd been there 3 weeks - and he valued us a lot more than them.