To those who said don't bother with revenge, rise above it etc.....IGNORE THEM!
Have some fun!
My first suggestion is obvious: (
disclaimer: you should not do these, it would be wrong and immoral. I am by no means responsible if any of the below occur)
paper bag with poo in it, leave on doorstep and set it on fire, then run and watch from nearby.
Then sign their phone number up to as many porno sites/free picture messaging sites as you can.
Superglue in the front door lock is a funny one, but a very harsh one
You can get valvecaps, or possible even make you own, that when they screw onto the valve depress it and release all the air. Let down a car tyre and replace with this valve cap
collect do poo in a bucket, then spread it all out infront of their front door.
show them the richp + crisps photo
paint their front door/garage door bright pink
expanding foam up car exhaust/in letterbox