I am amazed at how folks like those neighbors don't understand a simple 2-word phrase :

"OTHER PEOPLE"

Easy, isn't it ?

But then again, I guess they wouldn't if they were such incurably selfish assholes (which by the sounds of things is just what they are).

As for deliciously exotic forms of revenge, how about this (a variation on something already mentioned) : pee into a vial or a bottle (make sure you do plenty), and then pour the specimen down the windshield of their car so that it runs underneath into their car parts and air conditioning vents etc. ....