Can I cancel my ultimate after I've held down the left mouse button?

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:

You can use the right mouse button (or your custom key for movement) to cancel a skill cast at any point before actually casting it.

Using S (Player Stop Command) or H (Player Hold Command) will not cancel the ability.

I was going to put this in a comment, but it got too lengthy. In addition to the right click (what is obvious the best) as 3ventic suggested:

You can press esc. Escape also works when you click the ping button and want to cancel it. This is useful when you are recalling and want to use the shop. Left click would ping and right click would make you walk and cancel the recall. (That's not true actually, you can safely right click in the shop, it's just that I wouldn't risk it).

Another way to go about this is not to hold down the left mouse button. Simply click r to aim, and use shift+r (smartcast) to ult. This is a good trick to practice since smartcast is faster then left clicking.

Also, if you didn't know yet, you can use ctrl+r to level your ult. This can be a lifesaver if you reach level 6 mid battle

Last: on sona you can probably also use another skill instead of the ult. This would cancel the ult too. Not my favourite way since you might not want to use that skill at that time, but still an option.

My first point is do not let go of the left mouse button. If you do then your ultimate will be cast no matter what. Only let go after you've cancelled it. There are a few ways to cancel any of your spells after you've already held down the left mouse button:

• Right Click - this will cancel the ultimate without taking any other action.
• Pressing another skill - this will work if you want to use that skill anyway.
• Pressing esc - this can also cancel ping and closes most menus.

Some spells (but not Sona's ult) can be cancelled by activating it again but it will still go on cooldown, for example, Teleport or Pantheon's ult.

My personal suggestion would be to check the range by hovering your mouse over the ability button instead of pressing R.