welcome to germany. this is the easiest way, since they
have to accept the cancellation that way and i can directly tell them not to bother me about it ever again. they'd much rather i cancel by phone, but the place we're currently getting our internet from (vodafone) is notorious for lying to people on the phone and bugging you with offers and shit later*.
* which a) i don't want and b) they can stick up their butt no matter how cheap/good their offer, because they're incapable of supplying our future home with anything more than a 16mbit/s connection, which is garbage.
also i've already ordered proper gbit-fiber