My suggestion was a spell resistance that applied to the original save. Magic resistance makes it more difficult to cast the spell on a target.
So, if you cast a level 5 spell, normally the DC is 20 (10+2*5). A dragon has spell resistance 5 (made up number), so the DC to cast a level 5 spell directly on it is 25.
Cantrips are generally weak enough that spamming them won't do much. I mean, what's the danger? Ray of Frosting enemies to death at level 1? Using too much prestidigitation?