If you no longer want a website or app to remember your password, you can delete the password from the keychain where it is stored. You can also do this for certificates, secure notes, or other items stored in your keychain.

1) Open the Keychain Access app. The best way to do this is by using Spotlight.

2) If prompted to manage your passwords in the Passwords app or in System Settings, click Open Keychain Access.

3) If prompted for authentication, use Touch ID or type your user account password.

4) Click once on the desired item to choose it. Then, press the delete key on your keyboard.

5) If asked for credentials, type the username and password of an account with administrative privileges on your Mac. Then, click Modify Keychain.

6) Click Delete to confirm.

Outcome

The item that you chose is deleted.

NOTE: If you are using iCloud Keychain, the item is also be deleted on all of your devices signed in to iCloud, such as your iPhone.