If ONYX won't delete something, then it's something your system absolutely requires. Check out the advisories in the Help Center panel which should pop up on your Desktop when you open ONYX.
Googling the extension indicates that this is a known issue. The solution recommended is to reboot the Mac into Safe Mode with the Shift key, then open “/Library/Extensions†and look for “SUIDGuardNG.kext†and delete it. The cache will be rebuilt.
