Unauthorised subscription

We noticed that an amount of 164.85 *** US) went through our Credit Card. We had no idea what it was and how someone could just charge us without any authorization. our bank could not assist, they could also not even tell us what it was and if it was a monthly amount. They could only supply an e-mail address, [Email Removed] . I queried it and she informed me that we signed up for some mobile app. We definitely did not and I asked her to immediately cancel and reverse the amount. she just ignored me. We are pensioners and connot afford any amounts to be paid without any knowledge or authorization by us. PLEASE HELP.