There is one -- it's one of the buttons along the top -- but CSG subtraction is a bad mapping practice. The tool you really need to learn is the clipping tool. Select a brush, then shift+right click. You can define up to 3 points, which makes a plane. There are two buttons that look like a blue box with a red \ through them. One cuts off everything to one side of the plane completely, the other just splits it. There's one more button to the left of it that looks sort of like <-[|]-> That one switches which side you chop off. It's kind of like working with a block of cheese and using a knife to slice off chunks. Once you get the hang of it, it's about the only tool you need.