Are you thinking of joining KDP Select? If so remember its exclusivity clause, and make sure you give yourself plenty of time to have your book removed from other sites before you sign up.
It’s quick and easy to unpublish from Smashwords but there are also those that Smashwords distributes to. Unfortunately, it can take weeks for this information to filter through to these distributors. You may even have to contact them yourself to have your book removed from sale.