billboard liberation
n. A modification of a billboard or other advertisement that changes the ad's message.

Example Citation:
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.
—Karl Mamer, "Culture Jammers Expose Advertisers, Marketers," The Toronto Sun, April 12, 1998

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