When you complete an activity in the real world, it's very important to complete it in JobTracker as well. Otherwise, you won't be able to create meaningful views showing incomplete activities!
There are two ways to complete an activity - the first is by editing the activity and choosing Complete from the Status dropdown list:
Or you can click on an activity (on a job view, calendar view, or the job details page) and change the status from the popup menu:
Completing activities is one of the most important things you need to do to keep your data "clean" and help you make sure that activities don't fall through the cracks. If you've fallen behind on this, then set aside some extra time to clean up those old activities. That small investment of time now will return more time to you in the future as your scheduling becomes more efficient!