nostr:npub1zqsu3ys4fragn2a5e3lgv69r4rwwhts2fserll402uzr3qeddxfsffcqrs tested this for BTC Prague. Scratch-off stickers are pretty cheap, the problem is that it turned out to be pretty tedious to scratch the whole QR code area. Unlike other uses, where you just need to clear a number, you need to be very thorough so the QR code can be scanned properly.

We also toyed with other ideas, like crossing out the QR code after scan, or adding your signature, or using an origami-like folding technique where the QR code is initially not visible. Could be fun to explore this further.