If you plan to toss them. Use a strong electro magnet Should screw up the tracking (That's what we did with our old Magstar 3480/90 & reel tapes.) Or what our tape librarian suggested a sledge hammer. We charged $1 a tape on our United Way activity day, Was a interesting Activity. (Made more money then the hole-in-one activity).
If you plan to sell them (not sure what the market for used magstar tapes is) Have the vendor you sell them to certify the reformatting of the tape. Jim