What do I change in a physics asset to lock a bone in position?

I’m just a little bit lost in all options here. You’ve got the physics options, collision options, and body setup options, plus the options of all the constraints that connect them all.

Let’s say part of this animated mesh is literally screwed into the floor. I want the rest of it to flop around when I enable ragdolling, but this one bone should absolutely not move relative to the surface it’s attached to.

What’s the best way to accomplish that?