A key pair that has expired can be brought back into an operational state as long as you have access to the private key and the passphrase. To reliably render a key unusable you need to revoke it. Revoking is done by adding a special revocation signature to the key.

This revocation signature can be created together with the key. In this case it is stored in a separate file. This file can later be imported into the keyring and is then attached to the key rendering it unusable. Please note that to import this signature to the key no password is required. Therefore you should store this revocation signature in a safe place, usually one that is different from you key pair. It is a good advise to use a place that is detached from your computer, either copy it to an external storage device like an USB stick or print it out.

If you have not created such a detached revocation on key creation you can create such a revocation signature at any time choosing Keys → Revoke key, optionally importing it to your keyring immediately.