After a beach search left my sons raving about winning the "Stinkiest Shell" category in the Family "Best Beach Find Contest," we searched for a solution that would convince their mom to allow them to keep their Smelly discovery.
I found this tip (which I'm paraphrasing here) in a little guidebook from our library, Shellcraft Secrets by Greg and Shelley Stahly.
Removing a Smell from a Shell
1. Make sure there is no animal living in the shell. If you cannot tell whether or not an animal is still inside, it is best to leave it at the beach. Once the shell is home, however, and you are unsure if it is still inhabited, place the shell in water for a few days, being sure to replace the water each day.
2. Place the shell into a bucket of water with 2 Tbs baking soda for one hour.
3. Rinse with water and let dry.
Other techniques to try: Boiling the shell for 15 minutes. Freezing the shell overnight in your freezer. Or hanging the shell outside for several days -- the smell will eventually go away.