You can't hide or show a login button if the user hasn't logged in yet.
You could set the attribute user/Active false on registration and true when validated, but then unapproved users can't login and won't know why.
You could also assign a specific userrole on registration, one that can see or do nothing except login and be taken to a page explaining their registration has yet to be approved (or has been denied). And when approved, assign the proper userrole(s).