Cribbed from the net, good luck...
Soap and water
1. Fill a bucket with very warm, soapy water. Use a mild detergent, such as dish soap or a car-washing solution. Use a rag or dishcloth and scrub the offending paint. Depending on the vandals’ choice of paint, your problem may be solved right here. Keep in mind that the vandalism may just be a playful hoax with easily-washable paint. Regardless of the type of paint, if it hasn’t fully dried yet, this may do the trick. The sooner you try to clean the spray paint, the better. What could be a soap-and-water job could turn into something much more difficult if the paint is allowed to dry and bake in the sun.
Nail Polish Remover
1. Get any brand non-acetone nail polish remover. It's formulated to take the enamel layer off of finger nails, which is essentially what you're attempting to do with your car's finish.
2. Pour some of the liquid onto a terrycloth towel.
3. Rub gently to remove spray paint. It should take it right off.
4. Wash and rinse thoroughly when done.
1. Purchase a Smooth Surface Clay Kit, which includes detailing spray (which you would use as a lubricant for the clay), some wax and a microfiber cloth
2. Use the clay to remove the spray paint. It works very well and isn't harsh to your paint.
3. Wax your car afterwards
1. Purchase some acetone, a chemical solvent that can be purchased in large bottles for big jobs but which is also found in most nail polish removers if you just need to treat a small area.
2. Apply acetone to a small, inconspicuously located section of the damage. Either dip a rag in the acetone to apply it or dab it on with the brush included with nail polish remover. Wipe the surface clean immediately with a clean, dry rag, and wait a minute or two.
3. Continue incremental acetone application on the test area. If your car’s paint becomes discolored or faded, do not apply any more acetone. If your car’s paint seems unaffected, but the spray paint remains as well, try applying some more acetone and this time rubbing the affected area a little before wiping clean with a dry rag. The spray paint may come right off on the first pass, but it may take a little scrubbing and repeated applications, especially if it is thick in spots. The clearcoat finish on most vehicles is much more durable than spray paint, so you won’t easily damage it, but controlled, incremental testing in a small area minimizes the risk of damage and ensures that any damage that does occur will be small.
1. Purchase rubbing compound, which can be found at any auto parts store.
2. Use a dry, soft cloth and vigorously rub the rubbing compound on the offending paint. You might also try a slightly abrasive microfiber cloth (these can also be purchased at auto parts stores).
1. Try petrol. It might damage your clearcoat, but it will get rid of most spray paint.
2. Wax your car after cleaning, this makes it easier to remove spray paint if it happens again.
Brake Parts Cleaner (Spray)
1. If paint spatter doesn't come off with soap & water, but can be scratched off with a finger nail, try spraying brake cleaner on a clean piece of cotton rag & another to wipe cleaner off car. Rub 'cleaner' rag over small sections @ a time till removed. Wipe off cleaner.