Press Ctrl Z to undo your last action.
The hammer is in the tools list (Press Q and use the Mouse Wheel). The hammer can only destroy bricks that are exposed on either the top or the bottom. The hammer can not destroy bricks which would cause a chain reaction like the wand, making it ideal for fixing builds.
Type /wand into the chat (Press T) to activate the wand. The wand destroys any brick it hits. Any brick left floating will be destroyed. Be careful! The wand is best for destroying a group of bricks.
The Destructo Wand
This is the weapon of choice of most (abusive) admins. Use it on other peoples builds.