I can’t get spam from my calendar. Nothing is working
How to stop it from happening again
You stop it by not allowing web sites to send notifications.
If running iOS 13, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account.
If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account.