The two ways that I know are pains in the butt, but it can be done.
Method One (the wipe):
1) Before starting, back-up your BB data
2) Next, turn off only the calendar synchronization either on the server or desktop- wherever you set your sync options.
3) Wipe the Handheld (Options > Security > General > click for context menu > Wipe handheld
4) Type "blackberry" and watch all data be erased
5) Re-sync your BB with the BES, but this time the calendar sync will be off (per step 2) so you should end-up with a blank calendar.
And if you thought that was a pain in the butt, here's Method Two (but without a wipe):
1) On your handheld, set sync options to "mailbox wins"
2 ) In Outlook, move a copy of your whole calendar to a PST file
3) Open your calendar and set the view options to view by Catagory
4) Select all appointments and drag them to the Deleted items folder (this is important because the BB deletes items from the handheld based upon what gets moved to the deleted items folder.
5) Wait until you see the contents of you BB mailbox slowly removed one-by-one.
6) When handheld calendar is empty, Turn off calendar synchronization.
7) Move calendar items back from the PST (Step 2) to your mailbox calendar to restore all your calendar items.
8) With the calendar sync turned off, the newly transplanted appointments should not be replicated back down to the handheld
Hope that helps