How to disable select multiple categories in store module?

I search in module files but i not find...Meaby someone now how to enable only one categories select?

In modules\boonex\store\classes\BxStoreFormAdd.php

Find :

        $aCustomForm['inputs'] = array_merge($aCustomForm['inputs'], $aFormInputsSubmit);

Just above, Add :

        $aCustomForm['inputs']['categories']['attrs'] = array(
            'add_other' => false
        );