There are a number of items that will cause severe paint damage if used correctly (or incorrectly if you read their "precaution" labels

). Just a word to the wise, if you ARE going to perform anything underhanded, do it when things have "cooled down" and/or do it away from your neighborhood.....can't place blame if they can't prove it was you

Anyway the following will work to different degrees, and I will proceed from bad to worse...
1) Bologna.....Yes, Bologna!!!! The acid in the meat tends to eat away nice circular marks in other's clear coat.
2) Shaving cream- Will eat away paint after it freezes....can't take it off without taking off the paint
3) Brake fluid- Pretty bad, 'nuff said
4) and lastly, the god 'ole "key up and down the side of the car" trick....not exactly original, but effective nonetheless