We definitely have no control over having steam auto-install anything like that. There may be a glitch in the files where it thinks you were trying to download it and the download didn't finish or something, though. In order to clear out all cache stuff relating to any particular game on steam, you can do the following:
First, shut off steam.
Then, go to:
{SteamFolder}\steamapps
Under there, it has app manifests for a number of IDs. The AI War ID is 40400. You may see something called appmanifest_40400.acf or similar, and you can delete that. The AI War demo's ID is 40410. So if you see something like appmanifest_40410.acf or similar, you can also delete that.
If you then look under this folder:
{SteamFolder}\steamapps\common
Then you will see a variety of folders. Most likely there is an "ai war fleet command" folder. You can delete that. There may also be a file in there talking about AI War, or one of the IDs noted above. Those would be partial-progress download files, and can be deleted.
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All of that is just the standard stuff that steam uses to download any game. You'll see that for all your other games, too. My guess is that for some reason Steam thinks that it's not finished downloading the demo and is still trying to download it, so clearing the metadata about the game should also clear it thinking that it needs to download anything at all.
If that doesn't work, I'm afraid the ball is pretty much in Valve's court. You might google around and see if anyone has been having this trouble with any other games, and has found a solution other than what I've outlined. Usually the problem is with a download that has stalled and won't continue, and clearing these sorts of files makes it forget that it was trying to download so that you can start again. In this case I'm hoping that it will make it forget it was trying to download so that it will STOP downloading again!
Anyway, that's definitely a new one on me, but here's hoping that does the trick for you.
