Humm, I might try the Before event and see if the email is still sent, it's worth a shot I think.
Currently, we were using BizFormItemEvents.Insert.After += FormItem_InsertAfterHandler; and deleting it after it was created, but for some reasons, it didn't work once in a while...
I'll try the Cancel event, which seems the easiest solution to implement and test. As long as the email is sent, I'm fine with it.
Unfortunately, the site is very big and they have 20+ forms managed in the Bizforms panel. It's not currently possible to implement a custom solution due to restrictive budget and time.
But I'll try the cancel approach. I'll come back with how it went.