How to cancel a BC50?

I have a question regarding the cancellation of my BC50. My previous myBC50 was scheduled for renewal on 26.08.17. On 05.08.17, I went to the local DB office, where I requested for cancellation. The person at the counter told me that this is possible (incorrectly). On her suggestion, I bought a new BC25 with the same period of validity. Now, I get an email saying that it was too late to cancel my BC50. The representative at the DB office gave incorrect information, because of which I have two BCs. How should I proceed?