Try some Aeroguard, I watched someone easily remove some graffiti of a sign with some Aeroguard once, much to my amazement. Watching how easily it worked really made me wonder about what we are spraying on our skin, it didn't damage the sign but is probably worth checking in an inconspicuous spot first, he just sprayed it straight onto the graffiti, left it there for maybe 30sec and the wiped it off with a rag, wouldn't have believed it if I didn't see it for myself.