When viewing the “Trashed” tickets, the blue “New ticket” button becomes a red “Empty trash” button that allows you to delete all trashed tickets.
Note: the “Trashed” tab is visible only to admins and managers, agents don’t see it.
Regardless of this, all the trashed tickets can be automatically deleted after the selected number of days, as set in WP-Admin > Almighty Support > Configuration > Advanced > Cron > Delete from trash after: