Check out this thread.
Freeze Transformations (zeroing) on bones.
I'm not sure if the answer has changed since then, but I'm imagining it hasn't.
On a side note, for bones that I want to zero out, I'll often follow the procedure of control groups in Maya - take the bone you'd like to be able to zero out, duplicate it, make the new bone a child and now, if you need to zero it out you can. Honestly though, the workflow in the thread seems like it would be better.