In order for a salesman to go door to door here, he must have a peddler's permit. This may not be the same where you live. Also, there may be a law that gives you a certain amount of time to get out of a contract. I believe there is such a law here in New York State. I would think the Public Service Commission would be able to answer all of Al's questions involving the legality of it all.
People nowadays are very concious of older people being victimized and, it is possible the company may back off to avoid any publicity. Sometimes the local news station here will get involved and warn people that this type of thing going on. They will even name the company.
One idea is to post a sign saying "No Solicitation". This may or may not help. But, I don't think it would hurt.
Sorry you are having these problems. It is so hard when the roles are reversed.