Calendar Spam Notifications

I've started receiving spam notifications on my iPhone 6s. My calendar is not synced to my iCloud calendar although the fix mentioned to change your settings in the iCloud I put in last time this happened. So that fix seems to not be working.

Not sure if I've missed anything but note there's a new setting on the Calendar settings on the phone 'Show Invitee Declines'. This looks new and suggests by switching off I can safely decline the spam invites. Can anyone confirm? Or a indeed suggest an alternative solution.