windows 10 was moved to be recommended update in january and.. yes, you can stop it.
change windows update to not install updates automatically (or at least auto-install only critical ones) and let you select.
look for windows update KB3035583 and remove it. (it's the GWX update which puts the update to windows 10 pop-up to taskbar)
I'm pretty sure they update it regularly and give it a new number to get it to your computer yet again.
Also you can use taskmanager and look for startup things that point to said installer.
crude way to make it not succeed is to delete c:\$windows.~bt folder which is normally hidden and contains necessary install items. This should cause it to fail at trying to install itself.
in short, googling tells a lot.