Apple relies on users to deal with calendar spam
Before the “report junk” feature was added, if a person decided to just delete these fraudulent appointments, very often the endeavor would be counterproductive as this action informed the spammer that the account is active and usually this resulted in more phony requests. With the new addition from Apple this is no longer the case, but manually marking each invitation can be a rather time-consuming undertaking. Furthermore, the junk marking feature is currently only available on iCloud.com, but a future iOS update should enable this function for the iPhone and iPod.
source: BBC
Things that are NOT allowed: