I'm not aware of a way to completely disable the notifications, and to be honest, and as much as I hate to say it, the new MS Office for the Mac has a pretty outstanding Exchange client in Outlook. You might want to look into using that instead of iCal since you're so heavily into Exchange.
On the other side of the equation, we recently compiled the newest Darwin Server and have both a contacts (CARDDAV) and calendar (CALDAV) server up and running, sync'd to our iPhones, and shared across the LAN. It was completely free and works beautifully. I will say that Exchange is definitely a more elegant solution, especially if you need e-mail and shared e-mail folders, but if all you're looking for in calendaring, you might want to give Darwin a shot.