Spam issue in calendar. Can’t delete, it won’t allow me to. I need help

 If running iOS 13 or earlier versions, 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. 

Which version of iOS?

Then you need to follow the steps for iOS 14.

You are quite welcome.