You would have to set up a Supervisor rule with conditions based on the common pattern that you observe among the tickets in your Freshdesk account. To set up the rule, please navigate to Admin-->Helpdesk Productivity-->Supervisor-->New Rule.
A sample rule would be -
Conditions: Source is Email, Group is Unassigned
Actions: Delete the ticket.
Note: Make sure to Turn OFF the rule once all the tickets have been deleted.