Create an arsenal of cleaning products that can clean almost every kind of product (paint, chalk, markers) from every kind of surface (cement, wood, pavement). The best way to conduct a graffiti service is to offer a subscription-like arrangement. Once a month or whatever interval makes sense for your clients, go around to their property and clean off the graffiti. Charge them a monthly or quarterly fee and make it simple for everyone–they don’t have to think about graffiti, and you just do your job.