What version of exchange are you using? is it 2003? I know that 2007 did away with public folders but if this calendar is in a public folder you can setup the calendar as a moderated folder. then when someone posts to the calendar it will forward to you or whomever you set as the moderator for review then you can drag and drop it into the calendar. That last point is the possibly bad part. It will not show up on the calendar until whomever is set as the moderator checks it then adds it to the calendar. You can setup an auto response to tell the poster that it is under review though.