Some of San Francisco's top city and law enforcement officials met Thursday at an anti-graffiti conference to discuss the tactics the city uses to address the costly vandalism.

The blight of graffiti costs more than $20 million to San Francisco, its residents and private business owners each year, said police Chief Greg Suhr at the Zero Graffiti International Conference, which was convened by city officials and the international nonprofit Stop Urban Blight.

The spray paint is often used by gangs to claim areas of the city as their turf, Suhr said to a panel of law enforcement officials and other experts in the problem of graffiti.

San Francisco in the 1990s created an anti-graffiti task force and has since developed a database to catalogue the graffiti and determine who is responsible for each tagging, Suhr said.

Taggers often leave a "signature" on the graffiti and usually will not change it even if police can use it to identify the guilty party, he said.

Police make more than 250 arrests per year in San Francisco for graffiti-related crimes, and District Attorney George Gascon said the goal for prosecutors is generally to work with both the offenders and victims.

"We want a solution that works for everyone," Gascon said.

While penalties can include restitution to the victim and community service, Gascon said the community service usually includes education "to show them the damage they're doing."

Department of Public Works director Mohammed Nuru said the vandalism has since expanded more often to trees, sidewalks and even large trucks, allowing taggers to have "moving graffiti" in the city.

"We still have a long way to go," Nuru said.

One of the ways to prevent graffiti in the first place is to encourage other outlets for their creativity, said London Breed, who was recently elected to the Board of Supervisors after working as executive director of the African American Art & Culture Complex in the Western Addition neighborhood.

Breed said officials at the complex allowed youth to paint on the walls both inside and outside of the building.

"We have to give them alternatives to spray painting all over our beautiful city," she said.

The three-day conference, which started Wednesday, is being held at St. Mary's Cathedral. It will wrap up Friday with speeches by other experts as well as tours of murals that have been painted on public walls in the city's Mission District and Tenderloin neighborhoods.

More information about the conference can be found online at www.zerograffiti.org.