In order to prevent graffiti, you must learn to think like a vandal. Taggers select a target based on its buff/burn attributes. To buff means to clean graffiti. If you are a tagger, this is bad. To burn means to display with great prominence. Example: "My piece on the West Seattle water tower has been burning for a month!" So essentially they want a place where their graffiti can attract the biggest audience for the longest time.
Downtown Seattle is full of gleaming skyscrapers. Thousands of people walk and drive by them every hour. So how come they are never graffitied? Because, while they have a high burn attribute, they also have a high buff attribute. A downtown office tower is staffed by bored security guards with security cameras. If they see someone tag they will call the police. If the tagger is lucky enough to get away, the tag they left will be immediately removed by the custodial staff.
Just south of downtown Seattle is Chinatown. Perhaps a quarter of the buildings are abandoned or seriously neglected. These structures are covered in graffiti because they have high burn attributes, many can be seen by people driving on Interstate 5. And because nobody cares about them, the graffiti stays indefinitely.
They lesson is, clean up the graffiti promptly and they will give up. Another good tactic is growing vegetation over a problem wall or fence if possible. They will not tag vines or shrubs.