Is there a way to prevent appointment invitations from showing up automatically?

I have some users who doesn't want random .ics attachments creating appointments on their calendar. Instead, they only want appointments they put there to show up.

I'll anticipate one response right now: they show up as tentative appointments, so what's the problem? Answer: on iCal (which we use) even though it's tentative on the server, iCal thinks it's accepted.

Any guidance would be appreciated.