If you deface something, you've blemished or disfigured it in some way. For example, graffiti can deface a statue.
The de- in deface should be a clue that this is a negative word, but the face part could be confusing. This word uses face in a different way than usual — as if everything had a face. Think about regular faces for a minute — and how weird and silly someone would look with mustard all over their face. Not a great look, right? That's why we use deface when a building, a painting, a window, or anything that has been ruined in a similar way. Throwing a cup of coffee on a valuable painting ruins the face of the painting — so we say it's been defaced.