From the Portland Graffiti Removal program:
The most effective way to prevent graffiti from reoccurring is to remove it within 24 hours. This will greatly reduce and potentially eliminate the number of repeat "tags". The better maintained a building, the less likely that it will become a target. Call the police's GRAFFITI REMOVAL HOTLINE, 503/823-4824, to report graffiti and for more information about preventing and cleaning graffiti. Police reports are used to track graffiti crimes, and to identify building owners needing help. If you call to report, please provide the EXACT business name and address of the location needing the clean-up. Without this information, response time is lengthened considerably. (And when you report graffiti, it's usually best not to remove it till the police have had a chance to inspect and photograph it.) However, the law requires removing it within three days, and it also helps abate the vandalism if you do.
The Youth Employment Institute's Graffiti Removal Program (GRP) works year-round to help clean up graffiti throughout Portland. Operating with a crew of youth trainees, they paint over and remove and pressure wash graffiti from walls, signs, and other surfaces. This is a service provided through funding from the Portland Office of Neighborhood Involvement. The GRP focuses on helping those building owners who are unable to do the work themselves. It is not always possible for GRP to clean up graffiti within 24 hours, because many people request this service. Therefore, the best option for building owners is to do the clean up themselves, and as quickly as possible. GRP staff offers technical expertise and training for those who would like to remove the graffiti on their property, but are not sure how to go about it.
You can contact the GRP directly at 503/239-2856.