Hi,
The code you're looking for is in the file administrator/components/com_hikashop/classes/field.php on the line:
$string .= '<label><input type="'.$type.'" name="'.$map.'" value="'.$oneValue.'" id="'.$id.'" '.$checked.' '.$options.' /> <span>'.$this->trans($title->value).'</span></label>';
However, I don't recommend you to directly edit the code there as you'll loose the change after each update.
Instead of that, I would rather recommand that you do your view override file show_block_custom_item.php in Display>Views.
There, you have the code:
echo $this->fieldsClass->display(
$oneExtraField,
$itemData,
'data[item]['.$oneExtraField->field_namekey.']',
false,
' '.$onWhat.'="window.hikashop.toggleField(this.value,\''.$fieldName.'\',\'item\',0);"',
false,
$this->itemFields
);
which display each custom field.
So what you can do is change it to:
echo str_replace(array('<span>', '</span>'), array('', ''), $this->fieldsClass->display(
$oneExtraField,
$itemData,
'data[item]['.$oneExtraField->field_namekey.']',
false,
' '.$onWhat.'="window.hikashop.toggleField(this.value,\''.$fieldName.'\',\'item\',0);"',
false,
$this->itemFields
));
to strip the span tags from the HTML from the custom fields.
That way, no worries for future updates.