Studies show that graffiti removal within 24-48 hours significantly reduces the likelihood of recurrence. Home Depot sells un-wanted, mis-mixed paint at its paint counter for $5 a gallon, which is the easiest remedy to a tagged wall. For stone, cement, wood or brick (i.e., surfaces that you do not want to ruin by painting), I have found success with a slightly more expensive and labor intensive graffiti removal product (like this, which you spray over the graffiti and then scrub off). Of course, calling 311 is always a good option, especially when the graffiti appears on the property of another when you do not have that property owner's permission to take action against the graffiti yourself.
Below is the product of about 45 minutes of my de-tagging work this past Saturday. Removing or painting over graffiti our neighborhood is a neighborhood chore that I find really easy and gratifying, mainly because it's easy to see your own accomplishment. I plan to attack our alley this coming weekend, which has somehow accrued a lot of tags in the past month.