There's no direct option available to deactivate graph search once it is activated. But you can try a trick to turn this off.
As, the graph search is available only is "English (US)" you can get rid of the graph search by changing your default language.
To do so, go to 'Account Settings' > General > Language.
Click on 'Language' and select "English (UK)".
Click on 'Save Changes'
Now take a look, graph search has been disabled!