An Calendar event seems to be locked when participants has accepted the invite using Exchange?
This is a compatibility issue between Apple and Microsoft. There's not much you can do about that except using Outlook 2011 for Mac can help you out in this case.
As it is specific Microsoft software for Mac, you don't have all the limitations you have using the default (native) Mac Mail.app.

SideNote: A similar thing between Apple and Microsoft is when you flag an e-mail, it will be flagged as Outlook 2003, which is very pointless if your Exchange is 2007 or up.