I wouldn’t use those shells either. Even if you could clean the outside, the inside would still be filled with gunk. With shells so widely available from better sources, I wouldn’t put in the effort with the smelly ones.
OMG!!!!!!!! CLOROX??? NOOOOO!!!! The fumes will kill them!!! Where in the world did you buy shells with dried remnants of other animals in them? Did you rinse the shells long enough so no fumes remain? I would never rinse anything with clorox to be used with my crabs. The only thing I read on this website, was to clean tanks with vinegar and rinse thoroughly with water until there is no odor.