Heh, that's why I always fake address details when I'm checking out cable or DSL options. Of course they're going to use the personal information you enter for marketing purposes, that's why they ask for specifics like apartment number and your mobile number that they don't need to determine service availability.
It is possible to enter completely fake information, rather than someone else's address. (For example, aside from the burden on the post office, I think entering a street address for an apartment complex without an apartment number harms no one.)