Disable plugin and theme editor

The plugin and theme editors in wp-admin let Administrators edit code in the browser. That raises risk if an account is compromised, and makes accidental breakage easier.

Manual fix (Free)

1. Open wp-config.php in the WordPress root (FTP or host file manager).
2. Above /* That's all, stop editing! Happy blogging. */, add:

define( 'DISALLOW_FILE_EDIT', true );

3. Save the file.
4. Confirm Appearance → Theme File Editor and Plugins → Plugin File Editor are gone.

One-click fix (Pro)

1. Go to Security Ninja → Fixes.
2. Enable the option to disable the plugin and theme editor.
3. Confirm the editors no longer appear in wp-admin.

Related: Fixes overview, How to use the firewall for login protection.