Graffiti is the plural of graffito but has comfortably evolved into a mass noun and may be used with singular verbs. In fact, since the 1980s, the singular mass noun has far exceeded the technically-correct plural. Both the graffiti are and the graffiti is may be considered correct with one exception: if you use graffito in your writing, reserve graffiti for plural verbs. Avoid graffitos, which only serves to remind readers that the writer struggles to understand standard plural forms.