How to deal with payment scam through apple calendar?
Part of the solution, which fights the consequences not the reason, is to create another iCloud calendar named, for instance, "Spam" and move scam event(s) there without accepting/declining it (them). Then just delete this "Spam" calendar choosing 'Do not notify' when asked.

I think all this is due to hack of Yahoo mail when some people got access to many Yahoo addresses (about 500 million) and now they are sending those iCloud calendar scams.