A way to extract only one type of material during mining.

Set preference does not work since the excavators will keep digging even when the preferred material runs out.

Yes. Setting the preferred item to mine does not work.

Setting a preference does exactly that, it is a preference, it works just as intended.
I do however agree with this post, i would love to be able to restrict excavators to a set material(s). Then i could set some excavators to clear the soil and rock from the top of a deposit, leaving the juicy vein exposed below.