To be able to ungroup emails from a task, the 'show email details in lists' setting needs to be on so you can see the which emails are in the group. This setting can be found on the Sortd settings General Tab.
Once the detail mode is on, Tasks will show grouped email similar to the picture below. Hovering over any one of the email threads will show the little pop-out icon, which when clicked will create a new task with just that single email attached to it. this new task can then be deleted/archived etc like normal.
Note: Detail mode in Sortd can be slower than the less detailed mode especially if you have many tasks on lists, so feel free to switch between modes as necessary.