Woocommerce restrict based on the postcode

We have a product on our clients store, but this one product they only want it available in certain postcodes as its to heavy to ship.

I have checked these articles:

https://webcusp.com/5-easy-steps-to-restrict-shipping-buy-pin-code-zip-codes-woocommerce/

https://docs.woocommerce.com/document/woocommerce-conditional-shipping-and-payments/

That’s how I have it set, the problem is the product is still available on another shipping method, which we don’t want it too, i.e:

https://fair+++crisps.co.uk/product/25kg-sack-of-potatoes/

If you add to your cart and do to checkout and use this postcode on shipping: CO5 9SE

It gives you free shipping, but we don’t want that £4 delivery option available. that £4 is a custom shipping plugin that was already on the site which is what I think is causing the issue. That shipping is for the crisps on the shop not potatoes (I tried to deactivated it but the issue was still there).