I too got sick of the damn upgrade pop-ups. When I started the program today and the upgrade popup box appeared I saw the very slight appearance of something hidden at the very bottom of the popup page. The window shows full size, by height, so you can't move it up the screen to find out what the "other bits" are at the very bottom.
I had my task bar set on always showing though, so I changed that to autohide and I could see the boxes for "Don't show this again" & "Close"
I checked the "Don't show this again" and so far it hasn't