If you would like to programmatically approve a user's account you would need to do something like: 

            MembershipUser user = Membership.GetUser(UserName);
            user.IsApproved = true;
            Membership.UpdateUser(user);
Unapproving their account is similarly easy: 
            MembershipUser user = Membership.GetUser(UserName);
            user.IsApproved = false;
            Membership.UpdateUser(user);
Delete a user account:
Membership.DeleteUser(UserName);
And unlocking it:
            MembershipUser user = Membership.GetUser(UserName);
            user.UnlockUser();