The nagware recently arrived on Windows 7.
This "important" patch KB3035583 is driving the Windows 10 on-desktop promotions. Removing KB3035583 should remove the Windows 10 promotions. From the Microsoft website:
Update for Microsoft Windows (KB3035583)
This update enables additional capabilities for Windows Update notifications when new updates are available to the user. It applies to a computer that is running Windows 8.1 or Windows 7 Service Pack 1 (SP1). Before you install this update, check out the Prerequisites section.