Heres a little and simple guide on how to easly prevent steam from changing your CD key back to the old one.
1. Go to start and search 'regedit'
2. Navigate to, HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Bohemia Interactive Studio\Arma 2 OA
3. Right click the folder go to 'Permissions'
4. Click on advanced
5. Go to administrators and click edit
6. Find 'Set Value' and check the box 'deny' and accept changes
It now makes Steam unable to change the CD key.