There is an icon of an eye after the name of a (published) activity. By clicking this, you will hide the activity in the template and you will get the option to hide this directly in the existing courses.

You can show/hide activities by using the slider Show hidden activities and blocks. You can restore hidden activities using the icon of an arrow pointing left. When you use this, the activity will also be restored in the course(s). 

If you hide an activity that already has been finished by learners, the activity will also be hidden for these learners including the result on this activity. 
The result on this activity will be saved. So if you make the activity available again, the learner will see the result again on the Results page.