Are your business clients not running an Active Directory domain? A $400 copy of Server 2012 R2 Essentials will give them the ability to run an AD domain and with it, group policy. Then, with group policy, you turn off anything and everything you want related to upgrades.
I've got all that turned off, rarestia. That's not the problem. . . The users couldn't install Windows 10 if they wanted to. . . it's RAH-RAH pop-up ads that appear to be from Microsoft pushing upgrading to Windows 10 that interrupt their workflow and require them to hit a "go away button," which is very persistent and apparently is one of those that argues with the user. From what I can tell, they are not appearing on their normal browser or desktop, but rather on the proprietary browser/desktop of the vertical solution software vender. That, I have no control over, unfortunately, but the vendor claims it's not coming from them! Their system locks me out from doing anything by remote on their screen! HIPAA laws, you know. AAAARRGGHHH! I'm going to have to go look myself.