The Park Slope Civic Council really made a splash with their red laminated No Flyers signs. Turns out there's also polite sign to put in the window of a car that you are blocking. They're both available at the following locations:
Community Bookstore
143 7th Avenue between Carroll and Garfield
Lion in the Sun
232 7th Avenue near 4th Street
Brown Harris Stevens
100 7th Avenue at Union Street
Warren Lewis Realty
123A 7th Avenue between Carroll and President Streets
Orrichio Anderson Realty
302 7th Avenue between 7th and 8th Streets
Assemblyman Jim Brennan's District Office
416 7th Avenue between 13th and 14th Streets
Tarzian Hardware
7th Avenue between 2nd and 3rd Streets.
tb shaw realty associates
197 7th Avenue between 2nd and 3rd Streets
You can also download a "No Flyers, No Ads, No Menus" sign, a legally compliant (but less attractive) version of the sign, or a "Sorry Neighbor for Blocking Your Car" double-parking placard.