If your local cable company placed the cable behind your house, who owns it? Them. Do you expect them to let another company use their cable? Unless that other company places cable in your area, you're stuck with what is already there. They do not have to lease their cable to anyone else. Talk to the FCC on that one.

You're a captive audience.

As far a blocking, no way. If your local telco doesn't offer DSL in your area, either you're out of range (DSL is distance limited), or they just haven't upgraded their equipment, or they don't offer it for whatever reason. Call'em and ask.