When editing an activity (say, an Assignment), it is possible to set a grade condition and a completion condition for the activity itself. This way the activity could be made never available.
For example, add an Assignment in a course (where Activity completion is on as well Conditional activities) and set the name "Week 1 Assignment" . Save and return to course.
Now edit the "Week 1 Assignment" and click on either "Grade condition" or "Activity completion condition" drop downs. In both the Week 1 Assignment appears thus letting user to create never-happening conditions.