Unable to delete inappropriate invitations from Calendar

It sounds like you are syncing Calendar events from a third party provider such as a Microsoft or Google account. The synced calendar invite would need to be deleted off of those servers or the spam email where the calendar invite was added to those servers resulting in them being synced to the Calendar app.

Go to Settings > Apps > Calendar > Accounts and when the offending provider is prevented from syncing the Calendar events, they will be removed from the app.

In particular the Microsoft servers are regularly infiltrated with Calendar spam 2 or 3 times a year and their are frequent discussion over on the Microsoft Support site where that spam affects any platform where the Calendar is allowed to be synced. Just guessing on if you have a Microsoft account because that is the most common where this is seen.

If we knew the account where these Calendar invites originated, we may be able to provide additional information that does not involve just turning off syncing of that account and each provider has their own methods to resolve Calendar spam.