In The Truth About CreateSpace’s Free ISBNs, it was stated that if an author who has accepted a free ISBN from CreateSpace subsequently removes his book from CreateSpace, CreateSpace might reassign the ISBN that was assigned to his book (aka, "recycle" the ISBN). This is not true.
According to Amanda Wilson, CreateSpace’s Public Relations Manager, CreateSpace does not, and never has, reassigned its ISBNs. If an author accepts the free ISBN and subsequently removes her book from CreateSpace, the ISBN assigned to her book will go out of circulation.