I bet you have a plastic card or two from a bank. I mean, debit or credit cards like Visa or MasterCard. The vast majority of them (maybe, all?) have this CVV (aka CVC) code on the back side:
It’s very important for internet payments. Knowing it proves you’re the owner of the card (in theory). In practice, you need to remember or store it in a secure place, and then conceal it from the card.
One way is to scratch it away. But here is a less damaging way that works pretty well from my experience: first, apply some white correction fluid; then, sketch it with a black permanent marker.
Whichever way you choose, make sure other people can’t detect it, at least with a quick look.