Messing with billboards can get you in legal trouble, which is probably one big reason it hasn't happened yet, unlike ad-blocking on the web which is perfectly

Messing with billboards can get you in legal trouble, which is probably one big reason it hasn't happened yet, unlike ad-blocking on the web which is perfectly legal and doable. However, now that we have super-cheap small "drone"/RC aircraft, it would probably be fairly simple to rig one up with a can of spray paint and use that to deface billboards without being easily nabbed.... (Anyone who wants to use my idea is welcome to it!)