[otrs] Help please

Hello! I am new to OTRS. My intention is when the Ticket is already "closed successfully" by an agent, once the customer make a follow up, he should not receive any reply just notification that his ticket is already closed and he will receive new Ticket. But other way around happened. The OTRS Admin Account opened the ticket even the Ticket was successfully closed. What do I need to configure? Your help is highly appreciated. Regards, Len