You can't hide the menu bar on OS X from within Emacs. It's always shown for non-fullscreen applications, that's simply how OS X works. Applications have no influence on that.
No menu bar
If you'd like to use Emacs without any menu bar, enable fullscreen mode with
Removing individual items
If you'd just like to remove individual items from the menu bar, adjust the corresponding key maps in your
init.el. For instance, the following snippet removes the “Options”, “Edit” and ”Tools” menus:
(define-key global-map [menu-bar options] nil)
(define-key global-map [menu-bar edit] nil)
(define-key global-map [menu-bar tools] nil)
A mailing list post from Xah Lee has more examples.