How to disable all subsite using same user database/share user

Hi, may I know is there a way to disable user share in multisite.
Example, I need all my subsite user not related and user cannot use their Subsite A registered user to log into Subsite B. Currently, when user use their registered user email on Subsite A to register an user on Subsite B, there got an error "Error: An account is already registered with your email address. Please login."
So, may I know how to make my subsite to have their own user database but not share entire user database.
Thanks