Personally, when I couldn't stand MY downstairs neighbors, I used the water method to get rid of them.
The 'condos' (read: apartments) I lived in were 2 story and had free heat and hot water (this was in Anchorage, AK and that's common there). All of the hot water was tied into the same huige boiler for each building.
These people NEVER failed to flush their toilet while I was in the shower. For about 3 months, I endured blasts of cold water until I reached the peak of a Mt. Everest of loathing for them. The day I snapped was when I heard the guy say something on his telephone as I was coming out of the door and I realised he was doing it as a practical joke. Then my loathing turned to a cold hard ball of hate. I was now on a mission to out-asshole this fucker.
The pipes conducted sound really well, so you could always hear when your neighbors turned water on or off in that place. He'd been using that to get his kicks when I showered, by flushing the toilet deliberately. I made it my personal mission to go turn my tub on full blast, alternating pure hot and pure cold water every few seconds each and every time they got in the shower. I threw in flushes, just to make sure there was plenty of fluctuation in the temperature for them.
Strong in this one, the dark side is.
I got really good at it over the course of about a month. The time of day or night did not matter to me. I was always on duty, ensuring them not a single gallon of temperature constant water. I'm sure they became known as the 'stinky people' where they worked because unless they were sponge bathing, those people were not getting much shower time. 30 seconds, tops, before the water would just shut off.
They moved.