How to convince Apple definitively that I don't want to install iCloud?

If you click on "Remind me later" two times, it will change to "No thanks" on the third prompt:

[Screenshot of the "No thanks" button] (Screenshot source)

You won't get "No thanks" if you always click on the [X] to dismiss the dialog box.

The prompt won't happen again after clicking "No thanks".

Unfortunately, I was unable to determine where in the filesystem or registry iTunes stores the setting to display the popup for installing iCloud.

Go to start/Programs/Apple Software Update and check for updates... It will (of course) offer you iCloud for Windows again. Check it off, then go up top of the window to "Tools" and select "Ignore Selected Update(s)"... Done.