s2Member never restricts/removes access from the Administrator, so I'm not sure what you're experiencing there.
If you want to remove s2Member, it's as easy as deactivating the s2Member Framework and then removing the s2Member folders from the
/wp-content/plugins/ directory. If you also want all s2Member data to be removed from the database, then before deactivaing s2Member, disable this setting: WP Admin > s2Member > General Options > Deactivation Safeguards
I hope that helps.