You can respond to the attack, but not before the attacking party is depleted. I think of it similar to paying costs. The "cost" of the attack action is depleting all members of the proposed attacking party. Since your opponent cannot respond before costs are paid, they are unable to stop the attack by depleting a character. They can, however, play other tactics or abilities that might affect the attack in other ways.
The Gnomish One: Cracking Heist Planners in sealed since 2006.
~~Proud Member of Team Gandalf~~
The Garbageman can because he mixes it with love and makes the world smell goooood.