Can't remove bluetooth iPhone 3GS from OS X Mountain Lion
What if you just switch off Bluetooth? You're Mac will disconnect (obviously) and then you'll have removed your iPhone from your Bluetooth preferences.

If that doesn't work, you can do it the hard way and delete following files from ~\Library\Preferences:


com.apple.Bluetooth.plist
com.apple.BluetoothFileExchange.plist


Please be advised you'll delete all your stored Bluetooth devices !!