You can disable all alerts, including email alerts, in ical via
iCal --> Preferences --> Advanced --> Turn off all alarms
If you need to have iCal display alarms on the mac, but simply not send the email, then you will need to turn to other software. Here are two applications that help tame iCal notifications:
- This should allow you to define what emails get sent. It's primary use is taming meeting schedule notifications that iCal sends when you schedule a meeting with multiple participants, but it looks like it can be used to better configure what emails iCal sends
- This tracks your calendars (iCal, or google directly) and use growl to alert you for upcoming events. Use this if you want to disable emails by disabling all iCal alerts, and then setting this up to receive the alerts on the mac that you want to receive onscreen.