If the odor is off-gassing from one of the components of the pack construction you are SOL. It will always come back. You may be able to mask it somehow with a stronger less unpleasant odor.
I have a 40 year old set of Craftsman tools that has screwdrivers with plastic handles that off-gas a puke smell. I've learned to live with it and sometimes even entertain friends with it when I open my tool box on a hot day.