Invites can not be removed.
Forgive my lack of Mac knowledge, PC user through and through. But, have a client who is having trouble removing invites they created.
These are old invites that seem to have reappeared, now when they try to accept/decline, the notification reappears again within moments.
Towards the bottom of this bug :Bug 40541 – ical notifications box not functioning it begins to sound a bit like the problem the user is having.
osx 10.5.8 ical 3.0.8 (1287)
If you need more information let me know.