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if (! \Bitrix\Main\Loader::includeModule('iblock')) {
    throw new \Bitrix\Main\LoaderException('Unable to load IBLOCK module');
}

$property = \CIBlockProperty::GetList([], ['IBLOCK_ID' => $iblockId, 'CODE' => $propertyCode])->Fetch();
if (! $property) {
    throw new \Exception('No such property');
}

$query = \CIBlockPropertyEnum::GetList(
    [], 
    ["IBLOCK_ID" => $iblockId, "XML_ID" => 6, "PROPERTY_ID" => $property['ID']]
);

$value = $query->GetNext();
if (! $value) {
    throw new \Exception('No such value');
}

$delete = \CIBlockPropertyEnum::delete($value['ID']);
if (! $delete) {
    throw new \Exception('Error while deleting the property value');
}