Ad Jamming (also called adbusting) is a subset of Culture Jamming, focused explicitly on modifying and defacing ads. Ad Jamming can be as simple as the 1920s era style addition of moustaches to faces on ads, to elaborate billboard modifications. In general, Ad Jamming aims to subvert the ad and communicate a message contrary to the original. Examples include defacement of Cigarette ads to reflect health dangers, or changing the Nike slogan of "Just Do It" to "Just Don't". Ad Jamming itself, actually goes back to the Great Depression, with attacks on ads showing happier times, in an attempt at bringing a sense of realism to the images.
The most infamous group of Ad Jammers is the Billboard Liberation Front, which has defaced numerous billboards in California since the 1970s. However, other groups, and individuals have taken it upon themselves to modify ads to suit their own purposes. The BLF's jams are often so well executed that they are sometimes confused as an original ad at first glance. Among the BLF's best jams are a defaced Camel Cigarettes neon billboard which now says "Am I Dead Yet?", and the placing of Charles Manson as a spokes person for Apple Computers' "Think Different" ad campaign.
Ad Jamming differs from simple vandalism in that it aims to restore a public voice to the advertising that assaults the senses from all angles. It aims to spread a serious message, as opposed to just creating a bit of personal pride. As advertising continues to push further and further into the public consciousness, Ad Jammers will push back, using the media against itself in detournement.
Of course, advertisers being what they are, have taken Ad Jamming and tried to turn it back on itself with ads that appear pre-jammed, such as for the Dodge Neon, featuring an ad that appeared to have a "p" spray-painted next to the word "Hi". Actual jammers further modified these ads to say "Hype".
Of course, Ad Jamming is just one of many tools people can use against the corporate machines that permeate our lives. Pick up a book, or a marker and take some action.
Sources:
Naomi Klein - No Logo: Taking Aim At The Brand Bullies
The Billboard Liberation Front - http://www.billboardliberation.com
Adbusters - http://www.adbusters.org