Apply permissions per post

If you look at WordPress’s explanation of Roles and Capabilities, you will see that the correct role you should have assigned them is Author.

That being said, if you for some reason don’t want to change them to Authors, you can alter the capabilities that a role has. See the full list of Editor capabilities here.

remove_cap( 'editor', 'read_private_posts' );
remove_cap( 'editor', 'edit_private_posts' );
remove_cap( 'editor', 'delete_private_posts' );

This will permanently remove that capability, so after the wp-admin of the site is loaded once, you can remove these lines or comment them out.