Definitions:

Deck (noun) - The clean water tank of a toilet. Often referred to as a toilet backrest

Upper-deck (verb) - The malicious act of making doodie in the water tank of a toilet,
thus contaminating the toilet's clean water supply.

Syn: Upper-Tanking, Backstopping, 'The gift that keeps on giving', '2000 Brown Flushes',
'The bottomless glass of chocolate milk', Top-Loading, High-Bowl

Upper-deckerer (noun) - One who performs the act of upper-decking.

http://www.antiupperdeck.com/

I saw that phrase only the first time the other day and had to stop and think about what they were referring to... When I got it I was fairly revolted...

is there a similar noun for someone who urinates of balconys? ;-D