1.

Virgin billboard in London gets vulgarly hijacked.
That's what you get for lamely using Success Kid.
Origin headline: "Tim just realised his parents get HD channels at no extra cost."
2.

Right before the London Olympics, a group of artists brilliantly hijacked 30 British billboards.
Here, the famous Wayne Rooney Nike ad gets nicely busted by Bill Posters with a new tagline.
3.

*snicker*
Ad for "the pill" via Merck.
4.

Via NYC.
(Wikipedia) The Game is a mental game where the objective is to avoid thinking about The Game itself. Thinking about The Game constitutes a loss, which, according to the rules of The Game, must be announced each time it occurs.
5.

Erected in a very poor section of Bristol, England.
6.

Also via Bristol, where ad billboards are often defaced.
7.

A Tory billboard against then Prime Minister Gordon Brown gets hilariously edited.
8.

9.

A Brooklyn Bushmills board on purified Bedford Ave. gets nicely busted.
10.

"Fewer" adverts.
Still, that's the spirit.
Want more?
Here's 40 previous billboards improved by graffiti.