I assume you have a model associated to the shipping method.
In that module you must have a method called collectRates() that should return the costs of the available methods.
Just make your method look like this:

public function collectRates(Mage_Shipping_Model_Rate_Request $request)
{
    //return nothing if the shipping method is disabled from config
    if (!$this->getConfigFlag('active')) {
        return false;
    }
    //insert your logic here that determines if the method should be disabled
    //...and then
    if (your disable condition is met) {
         return false;
    }
    //rest of the code goes here
}

I assume you have a model associated to the shipping method.
In that module you must have a method called collectRates() that should return the costs of the available methods.
Just make your method look like this:

public function collectRates(Mage_Shipping_Model_Rate_Request $request)
{
    //return nothing if the shipping method is disabled from config
    if (!$this->getConfigFlag('active')) {
        return false;
    }
    //insert your logic here that determines if the method should be disabled
    //...and then
    if (your disable condition is met) {
         return false;
    }
    //rest of the code goes here
}

[EDIT]
The code you added does not work because the variable $item is not defined when you use it.
From your code I assume you want to not show the delivery method if at least one product in the cart has the value 0 for the attribute direct_delivery_product.
In case I'm right try this:

public function collectRates(Mage_Shipping_Model_Rate_Request $request)
{
    //return nothing if the shipping method is disabled from config
    if (!$this->getConfigFlag('active')) {
        return false;
    }
    $items = $request->getAllItems();
    foreach ($items as $item) {
        $productId = $item->getProductId();
        $product = Mage::getModel('catalog/product')->load($productId);
        if(!$product->getData('direct_delivery_product')){
            return false;
        }
    }
    //rest of your code here
}

Even if loading products in a loop is not the best practice, I think you should worry about optimizing later. This should make it work.