All I can tell you from everything I have read is cats DO NOT respond to punishment. The poop on the stairs was probably pay back. Cats can be very vengeful. Praise him when he is being good and for being inside. And I think you need to find a way to secure your screens more securely. Maybe a little 1 inch finish nail down through the plastic casing and into the runner would do the trick. Good Luck and I know it is often very difficult not to punish when they are bad but that absolutely never helps.