phone has been hacked

Ignore the warnings, they are Spam.

Then...

Remove Calendar spam

 In Calendar app, tap on Calendars and uncheck the ones you don’t recognise .

Then  Settings > Calendar > Accounts > Subscribed Calendars, look for the problem calendar and delete it.

PS when a website asked you to allow them to send you notifications, you said Yes and that is how you "Hacked" your own phone.