You can do this in the Explorer web client. Go to the Sites space under Company Home and "Manage Space Users", you will find the the EVERYONE group is a contributor, edit this so they are consumers. Then any users or groups you do want to allow to create sites just add them as Contributors.
In the UI everyone will still see the Create Site buttons and it won't be until they click on OK in the create dialog that they will find out they can't do it. So you may want to play with the UI to stop the buttons appearing for users without the permissions.