Disabled carrier become enabled after adding item in cart

[newcomer]

Hi guys.

 

I'm trying to disable a carrier, Carrier A, whenever batteries are added to cart because airlines wouldn't accept them. This is working so far as long as the cart only contains batteries, which Carrier A would be normally disabled. 

At this point everything is working fine.

 

However, when I added a non sensitive item to cart, Carrier A suddenly becomes available. Obviously this is a bug because it would force me to ship the batteries using Carrier A which I have not assigned it to.

Can anyone point me in the right direction to edit the php file that is exhibiting this behaviour?