How do I delete the calendar app completely?

Using iPadOS13 : Settings>Passwords&Accounts>Accounts... delete the rogue calendar you subscribed to.

Using iPadOS14 or later: Settings>Calendar>Accounts... delete the rogue calendar you subscribed to.

Regards

GIulio

The problem that you describe likely being very easy to resolve.

It is highly probable that an inadvertent click on a website link has “subscribed” an additional (unwanted) Calendar to your device - and this unexpected Calendar is exposing unwanted calendar events and sending you unexpected “adverts” or other warnings. 

Let’s check for what’s out of place...

iOS/iPadOS13 and earlier: Settings > Passwords and Accounts

iOS/iPadOS14: Settings > Calendar > Accounts

Look for an “account” that shouldn’t be in the list of accounts - as this will likely include the Calendar that contains all the unwanted events. When you find the suspect account, tap - then select Delete Account. This should resolve the problem in its entirety.

I hope you find this helpful in finding the source - and in resolving the issue if an unexpected Calendar is found.