For alfresco a content or a space is a children of a node.
So if your user have CreateChildren rights on space so he can create both.
Easy way to do what you want is to modify permission on "create_space" action and "create_space_wizard"
But that will just disable action, but with CIFS for instance, contributor will be able to do it.
Other way is to modify permissions definitions. I think you need to specialize rights for cm:folder. Out-of-the-box folder inherit rights from cmbject permissions definition.