I THINK...i think only, once they blacklist your CC they will not let you renew to gold next time on the same Gamertag. That's what i heard, perhaps someone can shed some light on this. Maybe they just block your future CC subscription.

Anyway, why you do not want to call them up & cancel? It's quite easy, Microsoft Customer Service ain't that bad...compare to those i've experience over here. (vodafone, broadband etc etc all their customer service outsource to India...)