Fotakill, requires root because it has to be installed as a system app, but I'm assuming you're rooted already if you're asking about it.
If it doesn't work after trying it, you can try renaming your otacerts.zip in /etc/security to otacerts.bak, that will also prevent the updates. When you do want to update just rename it back to .zip on the end.
Sprint GS3 Running TN's Msg and Chubbs