I've been playing Sona quite a bit, and I really do like the support role. However I've been missing her skill shot (her ultimate which is her stun) more often then I'd like to admit.
I noticed that if I press the "R" button (it brings up a sight to show me where it will hit) then press my mouse button the stun won't fire until I take my finger off of the button. So is there anyway for me to cancel it? mainly, because sometimes the enemy is already out of range or I made a mistake on where I thought they'd go off to.
I found an old gif on Sona's LoL wiki page showing it in action: