Q: An event appeared on my ical and I did not enter it myself.  Was it hacked?  How do I prevent it from happening again?

An event appeared on my ical and I did not enter it myself.  Was it hacked?  How do I prevent it from happening again? When I tried to delete the event, I got a window that gave me the option to delete but the sender would be notified.  I do not want any communication with this unknown sender.