If you are unable to revoke an old unused key, we recommend to disable that key.
If your friends are using the wrong public key when created messages for you and you are unable to decrypt them, they may be using the wrong public key. Ensure your contact is using the correct public key, see Verify a key. To avoid confusion, ask your friends to disable the wrong public key.
In GPG Keychain double click the key you want to disable. In the key details enable the 'Disable' option. The disabled key will no longer be used in GPGMail or GPGServices.