What does it mean to "hide' an Activity? It means that the Activity is no longer viewable to Account Holders and/or Instructors (depending on your settings).
Enrollsy has always allowed you to hide default Activities (except the Check In and Check Out Activities), which are:
Health Check
Photo
Note
Absent
Present
Here's how to hide custom Activities:
Go to the Activities page
Turn off the blue switch beside whichever Activity (default or custom) that you want to hide: