graffiti / graffito
The usual word, and the rare singular.
Graffiti is the usual word. It takes a plural or a mass-noun verb: The graffiti are gone. The graffiti is gone. Graffito is the rare singular. Prefer graffiti. For one mark, use a piece of graffiti.
Examples
Use The graffiti is gone.
Use a piece of graffiti
Avoid a graffiti