Apple - How can I hide or ignore old Outlook iCal invitations?

For an Exchange account the only way I found to clear old invites out was to log into Outlook Web Access in my web browser and decline them without sending an email in there. Just go to the same dates specified in iCal for the invites and use the right click menu in the calendar tab to decline them.

actually for me it worked to delete the caches: http://kb.mit.edu/confluence/pages/viewpage.action?pageId=12386508

It appears that several similar forum threads have your question and have had no success in finding an option in iCal to Ignore or Remove an invitation without clicking one of the options you have listed, which sends an email. Here is a workaround that is worth a try if you don't want iCal to trigger an email:

Use iCal Reply Checker. I've tried it myself and it works great to prevent or change the emails iCal sends when responding to an invitation. It is shareware, however, so remove all of the old events you need within the ten days that the application is free.