Patrick-Helpdesk
Set the ticket to a closed status that doesn’t allow reopen. Go to Admin Panel > Manage > Lists > Ticket Statuses, click any closed status (or create a new one), click Item Properties, un-check the box for allow reopen, and save changes. Now when you set a ticket to this status and a user replies it will open a new ticket instead of re-opening the previous ticket.
Cheers.