Daniele,
This may not match your use case, but if you had an attribute on the Task entity called Completed (boolean), you could use the following XPath:
Create a microflow that sets the Task.Completed boolean to true (be sure the set Refresh in Client to true). Call that microflow from a microflow button in the not completed Listview (button will be shown for each row in the Listview) or make that microflow an OnClick microflow for the not completed ListView.
Hope that helps,
Mike