Can't disable FaceTime auto-answering
It's a bit of a drastic move, but why not delete the preference files for FaceTime? This should put FaceTime in an uninitialized state (at least for preferences, perhaps not for configured accounts).

Close FaceTime, click on the Finder, then hit the Go menu and choose Go to Folder….

Enter ~/Library/Preferences into the box and press enter. Look for any files that start with "com.apple.facetime" and move them to another folder (for backup purposes).

Open FaceTime back up, check your preferences, and see if the Auto-Accept continues to occur.