I'm a pretty casual player, and like to play games I can (eventually) win ... and I agree that the Spell of Making victory is WAAAAY too easy. For you hard core players, I'm sure it's like taking candy from a baby.
I agree with the suggestions above to move the actual spell later in the tech tree, and make it cost more to cast.
I'd also like to suggest that you be able to cancel the casting once you've started it ... right now there's no way to stop it once it's started, at least not that I was able to find.
I, too, often turn it off as a victory condition, because it got so I was winning almost every single time with it on. Which, while winning is nice, gets a little boring after awhile. Ho Hum, I'll just gather resources and research stuff and then cast the magic spell and Viola ... I've won ... again. I've never tried to push it and see how quickly I could win with it ... maybe I will just for grins and giggles.