You can contact Valve's data protection officer at the address below.
While we review any request sent by mail, please be aware that to combat fraud, harassment and identity theft, the only way to access, rectify or delete your data is through logging in with your Steam User Account at http://help.steampowered.com and selecting the menu items -> My Account -> View Account Data.
Att. Data Protection officer
P.O. Box 1688
Bellevue, WA 98009
European representative for data protection questions:
Is there any requirement for them to provide an email address or name the DPO?