You'd need to get a proper lawyer to read the contract I would expect. As soon as its signed on your behalf then you are bound to it AFAIK, but I guess its not legally binding until both parties have signed it. This is why I always get the client to sign the contract and send the copies to me, rather than the other way around. There must be some "Termination" terms in the contract, and it would be those I would be looking at very carefully. In the case of my last contract, both parties had the right to terminate the contract without notice before the contract start date.
Im sure someone more knoledgable will be along shortly, but if not, try one of the many contractors forums, they will answer your questions quickly, but a lot of it will be down to the terms of contract