Is this a Time Capsule drive? If so then open Airport Utility, click on the Manual Configure button. Select Airport Utility Help from the Help menu and search for "erase." Follow instructions in the referenced article.
If this is a USB drive connected to an Airport Extreme Base Station then disconnect the drive from the AEBS, connect it to an available USB port, open Disk Utility to erase the drive.
If this is a third-party NAS product consult the user manual that came with it for instructions on reformatting drives.
Is the drive a pure NAS drive connected to your Airport via ethernet or a simple external drive attached to the Airport via USB? If it's the first you may (or may not) be out of luck. My understanding of NAS is that it is essentially a mini self contained computer with network storage function. Trying to reformat it would likely turn it into a brick. Perhaps check with the manufacturer to see what's possible.
You cannot use a NAS for a TM backup. You can use a USB drive connected to an AEBS, a Time Capsule, or an external drive connected directly via USB or FireWire.
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Format Network Drive
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