If you have a repeating activity on your calendar, you have several options for deleting the activity, dependent upon whether you want to delete one instance of that repeating activity, multiple instances or all instances.
Deleting One Instance of the Activity
As an example, let's say you have a daily repeating activity on your Calendar for the month of February 2013 - we'll call it Activity X:
Let's say you won't need to perform Activity X on February 13. Click Activity X on the 13th and you'll see the below:
To delete just this instance of the Activity, you'll want to select "This Date Only" at the top of the dialog and click Delete Activity at the bottom. This would leave all instances of Activity X on your Calendar, other than the instance scheduled for the 13th.
Deleting Multiple Instances of the Activity
Now, let's say you realize you will not need to perform Activity X from February 26th thru the end of the month. Click on Activity X on the 26th and you'll see the same dialog as we saw above, but we'll select a different option:
To delete Activity X from February 26 - February 28, you'll select "This and Future Dates" at the top of the dialog and click Delete Activity at the bottom.
Note: if the multiple instances of the repeating activity that you want to delete are not sequential date wise or they do not include all future dates, you would have to use the first method above to delete each of them individually.
Deleting All Instances of the Activity
If you no longer need to perform Activity X at all, you can click any instance of the Activity to pull up the dialog:
In this scenario, you would select "All Dates" at the top of the dialog and click Delete Activity at the bottom. At that point, all instances of Activity X will be removed from your Calendar.