Delete it. If you're on Windows, drag it to the recycle bin. If you're on a Mac, drag it to the trash. Do make sure you are deleting the one you want, first.
PGP Disk files are just files like other files. PGP Disk itself is an application that mounts what we call "container disks," and the PGP Disk file is the container. If you ask for a 10MB PGP Disk, then the file created will be 10MB (with a little overhead tacked on). It's pretty straightforward. If you delete or wipe the disk file, the disk is gone.
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This was first published in February 2003