Thanks to Kenneth Ives.
This program reformats the floppy disk, writes a file containing all 0s onto it, rewrites the disk with FFs, and reformats the disk again. See the code for details.
This method is moderately secure and should prevent data from being retrieved using normal data recovery software. It will not prevent serious data recovery experts like government agencies from recovering
some or all of the data. If you want to protect your data from everyone, shred and burn your disk.