With recurring tasks, you can either delete each occurrence of the task individually until you have reach the end of the recurrence cycle, or right-click on the activity, choose "Erase Activity" and then choose the option to delete all occurrences. Unfortunately, the second method can only be done one activity at a time.
Note: Effective 6/1/13, Sage no longers provides support for the Act! software. This is now provided by Swiftpage.