Some friends and I did something similar back in the '80s in NYC when between 2-5am we plastered strips saying "CANCELLED!" on thousands of Marxist AntiWar Rally posters two days before their huge rally. It really affected the turnout and the leftists never could quite figure out who screwed them.
I love that idea! I need to find some compatriots--all my friends just think I'm nuts to be so passionate about politics. Fortunately most of them are conservative but unfortunately they have not been moved to do anything. I did hear on the radio there are a group of Protest Warriors in my area so I may try to join with them. Keep up your good work!