One thing to keep in mind is that if you sign up for a BahnCard you'll automatically be enrolled in auto-renewal.
To cancel you'll need to write a letter (in German) stating you wish to cancel. I got dinged by this because I signed up for a trial BahnCard and forgot to cancel and was renewed for one year.
What I'd do is write the letter before you go to Germany and hand the letter over in the ticket office before your first trip.
Cancellations are for the auto-renew and not the term of the card.