How do you shut off this water softener bypass valve?
I don't know if you found an answer elsewhere, but I found a Youtube video addressing this issue.  The trick is that there are holes in the knobs where you can insert a screwdriver to give you more torque.  No need to pull out the knob or remove the clips.

https://www.youtube.com/watch?v=OKVvOCaGQJc