Yes, you can cancel your membership/contract.
If you no longer need advice and wish to terminate your membership, you must cancel your membership/contract. To do so, send a written notice of termination by September 30 of the year of termination to the head office of Aktuell Lohnsteuerhilfeverein (address: Aktuell Lohnsteuerhilfeverein e.V., Fischbräustr. 6, 84323 Massing, Germany).