if it is not possible to revoke an old unused key, we recommend to disable that key. This can happen if you forgot the passphrase for your secret key.
if your friends are using the wrong public key when encrypting messages for you, you will be unable to decrypt them. 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.
Open GPG Keychain and double click the key you want to disable. In the key details enable the 'Disable' option. The disabled key can not encrypt or sign new messages. You can still decrypt messages with a disabled secret key.