Hi,
The en-GB is the fall back language in both HikaShop and Joomla.
So if HikaShop/Joomla doesn't find a translation key in the current language, it checks if it can find it in the en-GB language.
So you can't disable the en-GB translation keys.
However, what you describe indicates that you probably have an error in the en-US language file or its override preventing Joomla from parsing the file and thus it can't use the translation keys of the en-US language, and thus it defaults to the en-GB translation keys.
I would recommend to activate the "debug language" setting of the Joomla configuration and check again the page where that translation key is used while logged in with the en-US language. At the bottom of the page, you'll see language debug information which will tell you which line of which translation file has an error.