If you want UDK to stop from telling Steam you’re running your game then disable the steam OnlineSubsystem with a simple INI file adjustment. Goto DefaultEngine.ini and add the following lines:
This should stop steam from notifying every person you know you’re running your game. I needed to disable Steam in UDK for a failed assert I got related to steam; unsure why it happens but it sure is annoying if it crashes your game while you don’t even use it.
Assertion Failed: Illegal termination of worker thread 'SteamFindServerThread'