I wear a One which doesn't have a band but my previous device was a BodyBugg that I wore on my upper left arm. It used to get a nasty armpit smell when I did a lot of exercise. I would either soak it for an hour in shampoo water or send it through the washer and dryer which would help. Eventually I even bought an extra band so I could wear one/wash one. I don't know if this will work for you but I hope it does.

I wash mine every day in the shower with some soap and water and thus have no had any issues. Try washing it daily if your not already.
Also you can pick up cheap bands on ebay Much better than having a hobo clinging to your wrist.

i have the old recalled force that i still love and refuse to replace- the band is fine, i never wash it, ive worn it thru all my workouts! no smell seriously!