The phone number of the Graffiti Task Force (Graffiti Busters) is 412-255-2872. They have a graffiti database which they use to try to track/identify taggers, so it might be productive to try and involve them. 311 is also a good place to report graffiti - and if it's hard to get through on the telephone, here is a link to the 311 web form: http://www.city.pittsburgh.pa.us/mayor/html/311_request_form.html