Pretty much there is nothing you can do... from the Steam website...
[blockquote]We will reassign ownership of the account to you upon verifying that you are the rightful owner.
If we are unable to verify ownership given the information you provide to us online, you may be asked to mail in your physical CD Key as evidence of ownership.
Note:
In the event that a hijacked Steam account is banned by the Valve Anti-Cheat System, Steam Support will not be able to reverse the ban.[/blockquote]