The answer to your question is going to depend on whether you're using the new gradebook or the old gradebook (classic):
- How do I mute or unmute an assignment in SpeedGrader? - instructions for the old gradebook
- How do I hide or post assignment grades in SpeedGrader? - instructions for the new gradebook
You can tell them apart by looking in the upper left corner of Speedgrader. If you have a bell icon, it's the old gradebook. If you have an eye icon, it's the new gradebook.