How can I hide or ignore old Outlook iCal invitations?

I often ignore invitations to office events on my phone, and then answer them later from iCal on my Macbook. Sometimes there are old invitations for events that have already occurred. However, the only options in iCal are Maybe, Decline or Accept.

If I choose any of these an email is sent to the requester which is confusing for them. How do I remove these from my queue without triggering an email? In other words, where is the Remove or Hide button?

Update 2013-01-08: The problem is that the invitations no longer appear on my phone (the "Invitations" list is empty) however they still appear in my Mac's iCal tray.

Solution 1:

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.

Solution 2:

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.

Solution 3:

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