You can easily copy the CryptoPunk contract and mint your own token[1], but the new token will be very easy to spot as a fake (as it will have a different contract address), and will have no market value, and likely zero buyers.
Compare this to physical prints, which can and often are forged in such a way that the original becomes very difficult if not impossible to detect.[2]
EDIT: The Banksy example you gave is one of impersonating the artist, rather than forging an artwork, and depends on the artist's accounts or website becoming compromised. It's not something NFT as a technology prevents; and, of course, the same problem could have occurred had it been a physical print sale distributed via the website.
Compare this to physical prints, which can and often are forged in such a way that the original becomes very difficult if not impossible to detect.[2]
[1] - https://etherscan.io/token/0xb47e3cd837ddf8e4c57f05d70ab865d...
[2] - https://www.smithsonianmag.com/smart-news/art-pranksters-sel...
EDIT: The Banksy example you gave is one of impersonating the artist, rather than forging an artwork, and depends on the artist's accounts or website becoming compromised. It's not something NFT as a technology prevents; and, of course, the same problem could have occurred had it been a physical print sale distributed via the website.