From what I read, if you log into icloud online and create a new category for calendar items and add the spam to that you should be able to delete the category and it should delete all the items in it.
None of the following is mine it has been pulled from a couple websites, but good info, I would have posed links, but this is my official 5th post, so couldn't until after this one.
Tip 1: Don’t decline the invite
Duh, I’ll just hit “decline” on the invite. Nope, not a good idea. Declining can notify the spammer that your account is active, meaning you could end up with even more annoying fake appointments in the future. Just ignore the invite.
The best way to delete one of these invites is to create a new iCloud calendar inside your Calendar app and label it “Spam.” (Open the app and select Calendars on the bottom of the screen, tap Edit at the top left, then Add Calendar under iCloud.) Once you do this, move the fake invites to your new Spam calendar and then delete the calendar.
Tip 2: Turn off iCloud calendar if you don’t use it
It took receiving a Ray-Ban sale invitation for me to realize I even had an active iCloud calendar account. I rely on Google for my calendar. If you do the same, or use Microsoft Exchange, Outlook or Yahoo calendars, turn off Apple’s iCloud calendar. On your iPhone, head to Settings > iCloud, then switch off Calendar.
Tip 3: Send iCloud calendar invites to email
If you’re an iCloud calendar user, you can opt to send invites to your email instead. This will prevent them from automatically being added to your calendar. Visit iCloud.com in your browser. Select Calendar then click the gear icon in the bottom left corner. Click Preferences then the Advanced tab, then head down to invitations and click “Email to [your email address].”
Remember, though, the next time you get an invite, you’ll have to manually add it to your calendar. Also remember: There is no such thing as $20 Ray-Bans.
Make a secret spam calendar and then delete it
If you decline the spam invitations, your response lets the spammer know that you're alive and checking this account -- which means you'll almost certainly get more spam in the near future. There is a way to delete the invitations without notifying the spammer, but it's a hassle. Here's what you need to do:
Open the Calendar app on your iPhone and tap Calendars at the bottom of the screen to see a list of your iCloud calendars.
Tap Edit in the upper-left corner of the screen and then tap Add Calendar. Name the calendar, choose a color if you like, and tap Done.
Go back to the main screen of the calendar app and tap Inbox in the lower-right corner to see the invitations you've received. Tap on a spam invite to open it and tap Calendar to switch the calendar. Move the invite over to the calendar you just created. Do this with all of the other spam invites you've received -- just remember, don't respond to the invite (by tapping Accept, Maybe, or Decline) or you will alert the spammer to your active account.
Once all of the spam invites have been moved to the spam calendar, go back to the calendar editing menu and delete the spam calendar. If you see a pop-up, make sure you press Delete and Don't Notify so the calendar will be deleted without notifying the spammers.
