do you mean can a local cable company scramble signals from local stations?
the technology exists but i highly doubt comcast would do this. if you're trying to get local channels then remember that they switched to HD which means you will need a HD tuner in your tv or the set top box you can buy for about $30.
or do you mean can your local cable tv company scramble their signal so you cannot pick up free tv?
i'm sure that measures are in place to prevent such things.