You notice that for some tasks the 'Right-click' context menu does not provide the 'Delete' option
ITMS 8.7
Any tasks that have currently running task instances that are not yet complete will not allow deletion. This functionality is by design as tasks cannot be removed if they have incomplete task instances.
The following query may show this condition:
EXECUTE tmCanDeleteTask @taskGuid='71f87ec3-89df-48a5-a4c8-455a64488b7f'
1 - True 0 - False