I think my phone has been hacked

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware, unless you have intentionally downloaded spurious softwares or unauthorised apps directly from internet and installed on your device or/and have Jail Broken.

It is calendar spam and not a hack.

See the replies at: Spam in my calendar - Apple Community

For future reference: That spam was added to your calendar when you agreed for some bogus website to send you “Notifications”.