[otrs] How - Stop re-opening of tickets by customer E-mail or interface?

Hello. Where do I set the parameters to stop re-opening of tickets when a customer sends an E-mail from their E-mail client OR sends a followup via the interface when the ticket state is closed (e.g. closed successful, etc.)? The system used to create the new ticket then send the autoreply (default closed ->new ticket), but somehow I changed something and not sure what. I have the 'Follow up Option:' => 'possible' Thanks in advance.