You may need to hold the mandrel in pliers or in vice grips while holding the bead with a scrubbing pad or jar opening rubber pad.
If this does not work, try soaking the bead and mandrel in water for a few hours. This often is enough to release the bead.
A little more drastic method is to then place the bead and mandrel in the freezer. After being frozen, the bead will most often come off as the water in the bead release thaws.
A final attempt can be made with a pop rivet gun. Insert the mandrel and operate the levers, and it will push the bead off the mandrel.
If all other things fail and you really want your mandrel back, you can warm the bead in the flame and dump it in water. It will break apart with the shock from the water. You can then clean up the mandrel for future use.