You can stop steam changing it via reg edit.
How to prevent steam from overriding your new CD-KEY
Hit your start button and type in run
In the run bar type in "regedit" *DON'T EDIT ANYTHING unless you know what youre doing*
Search for the folder Arma 2 OA , here:
Common Locations:
"hkey_local_machine/software/bohemia interactive/arma 2 oa" or
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Bohemia Interactive Studio\ArmA 2 OA
Note: you can use the search function to find "Arma 2 oa" - Edit drop menu, then search.
Right click on the folder arma 2oa that contains your KEY ,hit permissions, then advanced.
Select Administrators, hit edit, Change "Set Value" to Deny .. save
From
Change CD-Key to Arma 2 Operation Arrowhead - Steam Users' Forums