Can NFC tags be protected from being overwritten?

The Nexus S can write to NFC tags. How do you stop anyone with a Nexus S from overwriting your tag?

eg. If you have an NFC tag on a flyer that launches a video, can a guy with a Nexus S overwrite it and replace it with a link to his website?

Beginner here, just trying to get a handle on NFC, thanks!