i would thikn that bleach would do it or have you tried purple power??
i had a white banshee in 04' and here is what i did. i would wash it as best i could and then spray it down with Wesleys' Bleach White, and wash again with a soapy rag. this took every stain out and kept it looking brand new after every ride.appreciate all the tips....still nothing worked. I even took it to a couple of detail places and they couldn't get them.
A new set of plastics was around $120-150 last time I checked so I think I'm just gonna get a new set when I want it to look new again. heh. like that will happen.