The new Deutschlandticket is a subscription that must be cancelled in good time. You have to cancel the ticket by the 10th of each month otherwise you pay for it.
The most direct way to cancel is via the subscription portal under https://abo.bahn.de/portal/#/. There, rail customers log in with their bahn. de account. To do this, you have to specify a reason for termination and a termination date. Finally, you will receive a confirmation e-mail before the subscription is cancelled.