Adjust the query used as the Condition of the next task to consider criteria that would indicate the additional action is desired.

Example:
Our first action is a phone call asking to join your club. Once the phone call is marked as completed, you would like an email, the second action in the track, to be added automatically to the constituent's record if he is interested in more information.

Action Information, Completed equals Yes

After calling the constituent, you understand that he is not interested in further information and would not want the Action Track to continue to create an email action for him; however, when you mark the phone call action as complete, an email action is automatically added, despite the fact that you marked the phone call action with a status of Closed to note the constituent's disinterest in further information.

To stop the action track from creating a second action after the first one is completed, you can adjust the action query on the Conditions tab with the following filter criteria:

Action Information, Completed equals Yes
AND Action Information, Status does not equal Closed
 

By adding this filter, it will create the phone call action if email was completed but not closed.  It will not create the phone call action if the email was completed and closed, thus working with both scenarios.