10.21. I have some other problem I can't solve

In this case, if you know how to use GnuPG, you can try to achieve the same result through the GnuPG command line. For instance, if you cannot remove public key 0xABCDEF01 from Key Management, open a shell prompt and issue the following command: gpg --delete-keys 0xABCDEF01.

If the above doesn't work or you don't feel yourself enough experienced to use GnuPG, ask the friendly Enigmail/GnuPG community for support. References for obtaining support are listed in Section 13.