Disable Spellcast on the Minimap

This has messed me up more times than it should have. For example, I was just in a game, fighting as Nami near the top side of baron. The whole enemy team was nearer mid, the Braum shield and Yasuo wind wall were down, it was a game deciding fight, so I move my mouse to the bottom right of my screen, ready to ult them all and win the fight for us, and I ult our blue buff because my mouse was on the minimap. I have never intentionally used this feature, and never plan to. Why can't I turn it off?