Get the GENEVA demagnatizer; it will do cassette tapes and video tapes.
Usually you can demag about five cassettes then you have to let the unit rest a bit.
I have one and it works fine. I guess a wand will do it but it may take longer than the Geneva -- I'm not sure. I always thought a wand was a stick with a star on it and some little girl in a pick pintafore would wave it and turn me into a frog.