You can format an Eyefi card like any other memory card. Formatting will not delete any wireless network settings, but it will delete the Eyefi Center Software installer as well as any images on the card. If necessary, you can download the installer application here. We recommend formatting your Eyefi card in an Eyefi Compatible camera if possible.
If you format your Eyefi card in a computer, please use the Eyefi USB card reader. We do not recommended using 3rd party card readers that have slots for multiple types of memory cards (such as SD and CompactFlash in one reader). These card readers can cause issues similar to using Eyefi cards in CF adapters.
The SD Association has free software to format SD cards in computers, which we recommend using to format cards in a computer.