Back to Black washes away after a few washes and the wax stains will then return again. I used a white vinyl eraser, as suggested by somebody here (Thanks ), and it took away the residue permanently.
You can also use peanut oil, available at the supermarket. It's easier to use than peanut butter (especially when cleaning large areas) and , IMO, does a better job. Be sure to remove excess oil residue, it will attract dust.