How to remove Calendar event without the sender's notification
I found this free workaround over on Superuser.

Basically, you disable networking, delete the event, then delete the notification from email:


Turn off networking on your Mac
Delete the event from iCal
Wait for the notification to show up in your Outbox
Delete email from Outbox
Turn back on networking


Once networking is back on, the sync will remove the other events from iCloud and your phone.

Echoing Jaime Santa Cruz's comment below, you should definitely block the sender.  You have already confirmed your email because it (probably) already sent a notification that you accepted; best to be safe and not sorry.

As to your second question, "...it is enough reason for me stopping using the insecure Apple's calendar?" it would generate too much opinion to go into.

That said, I personally use Office365 Exchange Online ($48 for the year), Outlook.com, OneDrive and Azure for my cloud services with my iMac, MacBook Pro, XServe and (soon) a refurbished Mac Pro.  I don't know if that says anything...