As I have just seen on Daring Fireball, Apple has now released a first step to prevent any further troubles with iCloud calendar spam that appeared last month. Unfortunately this is currently only available on iCloud.com, but I suppose this feature will get rolled out in the macOS and iOS versions of Calendar in the near future. Or at least I hope so.
I haven't received any further spam invites so far and maybe that was only around the time of Black Friday, but in that case it's just a matter of time to happen again, I guess. Let's see, but in the meantime it's good to know that they have been acknowledging the problem and there's a first solution that has been rolled out.