I would guess the most effective thing might be to write a letter to someone at Tor (not the people in question, obviously) stating why you are buying X books, but not any others.

I'd be worried that not buying them in a way that would show up on the Tor budget sheets would lead to less support for those authors down the road when people are looking at the numbers.

I don't know how much attention the people at Tor would pay to such things, though - if they're sensible, a lot, because it is their company image being dragged through the mud. On the other hand, businesses are not always sensible.