How can we prevent the requester from reopening a closed ticket

As understood from your documentation "Closed" tickets will be reopened if a customer sends a rely on that. How can we prevent that and mark a ticket as permanently closed when no reply can re-open the ticket.

Refer competitor behavior for easier understanding between the "Solved" and "Closed" fields

Solved means that the agent has resolved the support issue. Solved tickets are closed, typically, a number of days after they have been set to Solved (the exact number of days depends on how an Administrator set this up). Until a ticket is closed, the requester can reopen the ticket. For example, the requester may not agree with the agent that the support issue is resolved and reply back to the ticket solved email notification.

Closed means that the ticket is complete and can't be reopened. Requesters, however can create follow-up requests for closed requests.