Preventing Baseband Update

1. Unpack Original/Custom IPSW

2. Decrypt Restore Ramdisk using xpwntool and mount it

3. Navigate to /usr/local/share/restore

4. Edit options.plist on the restore ramdisk

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
   <key>UpdateBaseband</key>
   <false/>
</dict>
</plist>

5. Reencrypt the restore ramdisk

6. Repack the ipsw

7. Restore IPSW to iTunes after preparing device for custom firmware.

This method works on iOS 4.2.1