When this first happened, I said "Nevermind" to make it go away but it doesn't understand that. I found that "Cancel" works.
So yesterday, I am driving and Google Now asks what I want. I say "Cancel" and then it says "Okay, the event has been cancelled."
WTF! This just deleted something from my calendar, possibly a co-worker's shared calendar, and I have no idea what the appointment was. I do IT consulting so I'm going to have an angry client on my hands if I don't show up and don't call because I have no idea I'm scheduled.