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.