How to get the smell out of a turtle shell

If you used chlorine bleach that will attack the bone itself and ruin it over time. Hopefully you meant peroxide. If you mean "nature cleaned" as in you found it rotting outside somewhere, try soaking it in straight ammonia for a week. Then soak it for a day in straight peroxide. That will also eat out a lot of stuck bits of tissue that are in and around the spine. Doing this might also lighten the scute plates that are on the outside of the shell but it will sanitize it and the stink should be pretty much gone after it is completely dry.