The least expensive way to erase a cassette tape is simply to record over it by turning down the sound level control to zero on a tape deck. There may be some remaining audio signal on the tape. For a complete, clean erasure of the tape, buy a bulk eraser from an electronics or computer store. Essentially a large electromagnet, the bulk eraser removes the magnetic signals that comprise the audio recording on the cassette, effectively wiping it clean. The process takes only seconds.
Place the cassette on a flat surface away from any other audio recordings.
Hold the eraser pointing downward, about three feet above the cassette.
Press the "Power" button on the eraser. Then slowly move the unit from one end of the cassette to the other and back again to the starting position.
Pull the eraser away from the cassette, then release the power button. This procedure wipes clean both sides of the cassette, so there is no need to flip it over.
Erasure is permanent and non-reversible. Keep the eraser magnet away from audio cassettes and stored separately when not in use.