I just installed Office 2011 on my mac and tried using Outlook for a little while. Unsure of why, it seems to have somehow put all the elements from my iCal configured Google Calendar into my work calendar running on Exchange. Not sure how this happened though, but I nevertheless decided to continue using iCal to avoid any more troubles (and deleting hundreds of private calendar entries is a big pain!).
Anyway, Outlook still continued to throw reminders at me, something which was duplicating what iCal was already doing. So I searched for a possible solution, but the search turned out less than fruitful. I still found out how to do it though:
* Open the reminder app: /Applications/Microsoft Office 2011/Office/Microsoft Office Reminders.app
Use finder or just use Spotlight to find it.
* Press «Office-reminders» in the menu and press «Deactivate reminders». Voila!
If you ever want to turn them back on, repeat the above process, just swapping deactive for activate 🙂