You could try the Mr. Clean Magic Eraser -use the extra strength one.
If that doesn't take it off, I don't know what will!
Or Would nail polish remover be a too toxic combination?
I guess we better be careful what we recommend. You know like not mixing ammonia and bleach; or recently I read don't mix vinegar and bleach! That produces toxic chlorine gas! Yikes.
Anyway try the magic eraser. That has gotten everything off for me.
"If you walk the footsteps of a stranger, you'll learn things you never knew. Can you sing with all the voices of the mountains? can you paint with all the colors of the wind?"