How do you erase a cassette tape?

The easiest way to do this is to record over it, but there will still be residual audio (or video if a VT) from the previous recording. If you wish to erase a confidential recording, I would recommend a Tape Degausser. This emits a rapidly alternating electromagnetic field which demagnetised the tape and thus erases it, in much the same way the degauss feature works to demagnetise components in CRT monitors. However, Tape Degaussers can be very expensive.

If it's important it be erased, with no trace and minimal cost - throw it on the fire, but you'll be unable to re use it afterwards.