I have been experimenting with fresh installations of Windows 7 and Windows 8.1. I agree with saltgrass that it is thoughtless of MS to provide such a convoluted method of accepting or, worse, stopping the upgrade. I found that unchecking/not checking the ofer to upgrade, in the Windows update, was not enough. On reboots, it still attempted the downloads. I tried my own suggestion, and found it did not work.
I explored the MS TechNet pages and, in the end, came up with this "final" solution.
First. Open the "Programs and features" and, over on the left, the update history. Look for these three files, and uninstall them. You will probably find you only have the first, as the other two are part of the "accumulative " update. Now, this is important. You must have your update set at check but do not install. The next time , which will be almost immediately, you will see these same (or just the first) updates offered again. Thios time you right click and "Hide" them. They will not be offered again.
KB3035583, KB2990214, and KB2952664.
Now, open the Task manager, scroll down the processes and stop/terminate, the GWX.exe process
Now go to the Windows\System32\GWX.
You must rename the GWXUXWorker.exe and GWX.exe to .old (Eg. GWX.exe.old). You will find you do not have permission to do this, so it is necessary to open the properties - Security and change the security to a lower level. This is a simple process, but a little work required. If you are not knowledgeable in this procedure, I or another member will advise in another post. It is NOT for the amateur.
Reboot and you are done.
The usual proviso. Make and image or backup before attempting the above.