Privacy hedges are a great long term graffiti deterrent for walls highly targeted by vandals. If you have a wall repetitively hit by graffiti vandals you can bet there's a lot of traffic that passes by the area.
Graffiti vandals want their tag seen by as many people as possible, so they'll scout out areas that will be best suited for their needs. Properly placed hedges will reduce the visibility taggers are looking for. In some cases shrubs or hedges have eliminated graffiti completely. In the picture below you can see how one man in Detroit, used this idea to combat a graffiti problem with 100% success.
When you choose to use shrubs or hedges as a method to deter graffiti vandals you may want to consult with a local nursery first. You'll want a variety that will survive well in your area. Typical you want a fast growing variety that will cover the area quickly, most fast growing types may require more trimming to keep the appearance pleasing. The cost isn't comparable between trimming shrubs/hedges and repetitive graffiti removal. Over a short time you can eliminate graffiti from a normally highly targeted area, saving you every year from spending money on graffiti removal.