The text input should detect private keys and reject them with an alert for the user.
A similar thing happened to me recently. I just confused the input fields. Luckily I just sent my public key per memo.
In my opinion this is an UX issue. It is too easy for the user to confuse keys. Generally, Steemit needs to educate people more about keys. Knowing how to manage multiple levels of private and public keys is not laymen's knowledge.
Bravo, that is the only thing which could help and should be implemented.
Even I did a similar mistake. Thankfully changed the password immediately.