I started getting these earlier this week and had no idea how to get rid of them. There's not a "No" or "Fuck You" button, so your only real option is to hit Decline - letting the sender know they hit upon an actual person, and guaranteeing the spam will only increase.

Luckily the internet came to the rescue, and Twitter user Aaron Douglas wrote up a nice blog post detailing how to get out of calendar invite hell:

Preventing iCloud Calendar Spam Invites