I know this is a little old, but I just want to add that the last version (downloaded today) of the Securom diagnostics tool has allowed me to revoke the licence of Arkham Asylum Goty.
The download is here:
https://support.securom.com/diaghelp/download1.html
After install it, I searched for applications with securom in the drive where I have steam installed, some of them appeared. Then I selected Arkham Asylum and went to the menu "application" and hit "revoke", a little window appeared telling me that the application has been installed with the serial "..." (insert there the serial of you installation, i checked mine and was the same shown by steam), and offering the possibility of doing and online revoke or a manual revoke, i clicked on "online revoke" and after 2-3 seconds, a message appeared telling the licence had been erased.
I have used the diagnostic tool in the past and I don't remember it having the option to "revoke", so I thought that I should tell my experience, maybe it can help somebody.