How to do it -
- Navigate to your activity
- Go to "Activity Settings" at the top of your page
- Click on "Build Manually"
- Go to "Grading" at the sidebar on the left of your page
- Turn of "Auto-Grading"

Note: This turns off grade visibility for everyone, including the activity owners. Qualitative feedback in the form of strengths, areas of improvement, and follow-up ideas will still be visible.