Go to My Cards in your dashboard and press Deactivate next to the card you have lost. It stops working immediately, and anyone who taps it gets nothing — not your profile, and not your name.

What happens to the card depends on how you got it, and the difference matters:

A card you bought stays yours. Nobody else can claim it, even while it is switched off. If it turns up down the back of the sofa, switch it back on from the same screen. A promotional or giveaway card becomes claimable by whoever is holding it. You can still reactivate it later, but only if nobody else has claimed it in the meantime. The screen tells you which of the two applies before you confirm, so read that line if you are not sure.

Deactivating one card does not affect your other cards or your profile. Everything else keeps working exactly as before.