Spam in Apple Calendar
First, do not "decline" or "accept" the invitation. It tells the spammer that your account is active.

Next, you need to make a change to your iCloud account.


Go to icloud.com
Login to your iCloud account 
Navigate to the Calendar App 
Click in the settings (gear icon) in the lower left of the screen 
Select "Preference" from the menu 
Select the "Advanced" tab 
Under the "Invitations" section select the "Email to" options


This will not stop the spam, but will forward it to your email, so you can deal with it there and keep it from getting to your calendar.

You can deal with the invitations already on your calendar by moving them to a new calendar and then deleting that calendar.


Create a new iCloud calendar (not “On My Mac”).
Move the spam event to the new iCloud calendar.
Delete the new iCloud calendar.
Calendar will now prompt you with “Delete and Don’t Notify” and “Delete and Notify”.
Select “Delete and Don’t Notify”.