The easiest, cheapest and safest method to eliminate graffiti from vinyl siding is simply to paint over it. Depending on the original color and paint type on the siding, paint either just the graffiti or apply a whole new coat to maintain consistency. If the paint does not sufficiently eliminate the graffiti, a chemical solvent may be needed.
Many solvents that effectively clean graffiti may contain chlorinated hydrocarbons and other harsh chemicals. So while chemical solvents can actually clean off the graffiti, rather than just cover it, the chemicals can easily damage the vinyl as well by removing its outer coating. When picking a solvent, first test it on a small area of the vinyl or on a spare panel to gauge both its effectiveness and its risk of damaging the siding.
Rub the solvent on with a non-abrasive material that does not scrape the vinyl, such as a cloth or sponge. After cleaning the graffiti with chemical solvents, it may still be necessary to repaint to mask any discolorations. If the building is in an area where graffiti is a common problem, consider placing a protective coating on the siding that will expedite the future cleaning of graffiti.