You can't completely get rid of the little upgrade thingy unless you find and uninstall the "important" Windows Update that placed it there. Requires some spelunking among the hacker and MS tech support sites to find out which. However (assuming Windows 7; 8 probably differs in some ways):

If you've already "reserved" a copy, you can bail on that. Click on the update applet; once it opens, there's a "hamburger" menu icon in the upper left. Click on that. Click on "View Confirmation" on THAT screen; there's a link for "Cancel Reservation" - do what comes naturally. It'll ask you to confirm cancellation at least twice.

And...

Absent uninstallation, you can also "hide" the icon if it really bothers you. Click on the little up-arrow in the right side of the taskbar - that's where notifications that you don't need to see all the time are hidden. Click on the "Customize" link. Choose "Hide Icon and Notifications" for the "GWX" applet. From now on, it should stay in that hidden up-arrow folder unless MS gets too pushy.

I did discover that you can download W10 as a .iso file and burn it to a DVD for later use. Much later, for many of us.