You can delete your GitBook user account at any time. Be careful, this operation CANNOT be undone.
Before you do so, you should transfer ownership of any organizations you might own. All books owned by your account will be lost, so you might want to transfer them to another user too. However, discussions and comments you have made in other people's books will remain, becoming anonymous (transfered to a "Ghost" user).
Your username will become available to new accounts, and any billing you had for paid plans will be canceled.