If you deface something, you've blemished or disfigured it in some way. For example, graffiti can deface a statue.
To deface something means to damage it or just mess up its appearance. And sometimes it’s both: Throwing a cup of coffee on a valuable painting ruins the face of the painting, so we say it's been defaced. Drawing a mustache on a painting is another way to deface it. What did the Mona Lisa say to the museum visitor holding a marker? “Don’t mess up deface!”