What a useful link, thank you very much.

Here is my version of a cancellation letter, but someone else may have something better and I tend to change them all the time, not employing the same model all the time. Above the BahnCard-Service address you need to put your address, usually top right. You can play with and spread the formatting to look better, naturally. I try to keep these letters as short as possible, although for some it may appear a bit curt. Lastly, when you write the date a full stop always appears after the number. The months of the year in German are: Januar, Februar, März, April, Mai, Juni, Juli, August, September, Oktober, November & Dezember, so you can just substitute the appropriate one into your letter.


BahnCard-Service
60643 Frankfurt am Main

16. November, 2014


Betrifft: Kündigung meiner Bahncard 25

Bahncardnummer 7081 xxxx xxxx xxxx


Hiermit möchte ich meine Bahncard 25 mit der Deutsche Bahn kündigen, bevor sie am 3. März 2015 verlängert wird.

Ich bitte um schriftliche Bestätigung.


Mit freundlichen Grüßen

Concerto

EDIT to add: I think it's always important to ask for a written confirmation (Ich bitte um schriftliche Bestätigung).