To disable the Google search bar go to Settings>Applications>All Applications, scroll down to Google Search and disable the app.
This will remove the Google search bar from the home screen.
To re-enable the search bar, go to Settings>Applications>All Applications, then scroll to the bottom of the list. This is where you will find all your disabled apps. Then select the app you want to re-enable and then press Enable.
NOTE: I am unsure as to how this will effect the global search functions. The browse and other in app searches should work fine.
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