Smashing the Image Factory - A complete manual of billboard subversion and destruction. Humorously ranges from the innocuous to the terrorist. - http://www.eco-action.org/dt/smashing.html
Ron English POPaganda! - Prolific producer of surreal and parodic billboard art since 1982. - http://www.popaganda.com/
Billboard Liberation Front - "Improving" outdoor advertising since 1977. Includes how-to manual for altering outdoor advertising. - http://www.billboardliberation.com/