<?php

require_once 'app/Mage.php';
Mage::app();

$categoryId = '';
$attributeCode = '';
$attributeId = '';
$products = Mage::getModel('catalog/category')->load($categoryId)->getProductCollection()->addAttributeToFilter($attributeCode, $attributeId);

if($products->getSize() < 1) {
    echo "None products found.\n";
    exit(0);
}

Mage::register('isSecureArea', true);

foreach($products as $product) {
    $product->load($product->getId());
    echo "{$product->getSku()} - {$product->getName()}\n";

    try {
        $product->setManufacturer(null)->save();
    } catch (Exception $e) {
        echo "ERROR: {$product->getSku()} | {$e->getMessage()} \n";
    }
}

Mage::unregister('isSecureArea');