To delete a Recurring Task you will first need to cancel the recurrence and then delete the task using the Delete Task and Purge Data Utility. 

To stop the recurrence of a Recurring task you will need to cancel the Recurring pattern. 

To cancel the recurring pattern, do the following:

1. On the My View Page, click the Utility Gear .
2. Go to the Task Utilities > Manage Recurring Tasks utility.
3. Select the Edit Recurring Tasks radio button.
4. Search for and select the recurring task needing to be cancelled.
5. Click Edit Recurring Task.
6. Under the Period End Recurrence Pattern section, select the Cancel Recurrence radio button.
7. Click Apply.