billed for service never provided!

I signed up for DSL service, but when I got the modem it wouldn't work. After long time on line with the tech people, I was told that actually DSL is not available in my neighborhood. (They had discovered this when they tried to connect it, but nobody ever told me) I immediately cancelled the account, but AT&T is billing me for $73 for account setup and for the couple weeks I supposedly "used" the service. Also I never got the $50 cashback coupon that was supposed to cover the account setup.