I just received an answer from the XBOX-Live email support. A guy with the name Thomas is telling me that it is not possible to cancel xbox live by email or snail mail. He refuses to tell me a postal adress of someone responsible at Microsoft. It is unbelievable that a corporation like Microsoft believes to be above all laws. IANAL but I am quite sure that refusing to accept postal cancelations or cancelations without the credit card number that initally paid the account, is pretty much a violation of german law.