I have not tried it yet but someone showed that the Mr. Clean scrubbing pads will take them off. But I found this and believe this is what I did something similar.
1. Heat the iron to hot (cotton or linen setting).
2. Lay the damp terry cloth towel on the ironing board and rub the hot iron vigorously over it to remove as much residue as possible.
3. Lay the sheet of waxed paper on the ironing board and liberally sprinkle with salt. Rub the iron in a circular motion on the salted waxed paper.
4. Rub the iron over the damp terry cloth towel again. Repeat the process of scrubbing with the salt and rubbing on the towel if necessary.