title: Block OpenSea trading
body: <p>Is there a way to avoid trading NFTs on standard marketplaces like OpenSea without breaking the erc721 standard?
  If so, how would you go about it?
  It is about an NFT that is something like a voucher that can be used 5 times. Over 5 years, once per year. I would like to prevent that someone unknowingly buys a redeemed voucher (for the current year).</p>