If nothing else works, you could always:
1. Copy/move your games (steamapps folder) to outside the steam directory.
2. Uninstall/remove the steam installation.
3. Reinstall.
4. Copy games back into directory.
This worked for me when I had to reinstall once. If this still doesn't remove it, then you'll know it's external and can probably be caught by other malware/adware scanners.
