n. A modification of a billboard or other advertisement that changes or subverts the ad's message.
Filling cars with cement? Maybe, but that could alienate the general public. Chaining themselves to various buildings? Definitely, but they first must figure out which "lock-down" technique to use. An aerial assault, possibly some parachutes? Get real. Billboard liberation? Oh, yes.
But these particular young rabble-rousers were also anarchists, nineties-style, congregating to parse strategies of "culture jamming," "billboard liberation," artistic media manipulation and alternative economics.
Billboard liberation amounts to property destruction and I personally can't advocate that kind of behavior. But I must confess I do get a chuckle when I see a bus shelter ad featuring a too-thin underwear model turned into a ghastly skeleton with no more than a few simple strokes of a black magic marker.