If your camera has a write protect key (it may be called Lock or Protect), you can use it to select which pictures transfer from your Eyefi card to your phone, tablet, or computer. The way your Eyefi card transfers write-protected photos depends on the type of Eyefi card you have.
Eyefi Mobi cards have a Priority Transfer feature. That is, if you write-protect photos, your Eyefi Mobi card will first transfer the photos you've protected and then transfer the unprotected photos.
For more details, please see the Priority Transfer article.
Mobi Pro cards have a Selective Transfer feature. After you enable the Selective Transfer feature in the Eyefi Mobi Desktop app, your Mobi Pro card will only transfer photos you've protected.
Note: By default, your Mobi Pro card has the uses the same functionality as an Eyefi Mobi card's Priority Transfer feature described above. You must enable Selective Transfer if you wish to use it.
For more details, please see the Selective Transfer article.