If your studio keeps a card on file for you, they can charge it only for purchases you ask for, you get a receipt every time, and you can remove it from your Appointments page whenever you like.
Three ways: you add one when booking an appointment; you scan a code or open a link from the studio and enter it on your own phone; or you ask the studio to save the card you read out to them, and they tick a box saying you said yes. In every case the card is held by Stripe — the studio and Klasly never see the number.
Charge it for something you asked for — a pass, a session, an add-on. Never automatically. Each charge sends you a receipt naming the amount, what it was for, and who at the studio charged it. If a charge looks wrong, reply to that email.
Appointments (My Appointments) → "Card on file" at the top → Remove. That also withdraws your agreement to charges. Anything already charged is unaffected, and you can add a card again any time.