It’s been a while that this issue has been ongoing and Apple hasn’t been able to fix it yet. An Apple spokesman apologized about it.
On iCloud’s calendar app, when the Report Junk button is pressed then a new window will open and it will be confirmed from you that whether is this invite spam or not. If you confirm it then it will be marked as spam. In future the invites from the same sources will also be marked as spam and they won’t show up in your iCloud Calendar. This button and feature will help Apple to improve its spam filters which caused this problem in the first place.
With the help of iCloud’s calendar web app you can remove all these spammy invitations and won’t receive invivations from them again.
According to a Reddit user tamag901, the Apple reprsentative told him that the Report Junk button will also be added to calendar app on iOS too and it will most likely come with the iOS 10.2 update which is expected to release this week.