Unmounting an iDisk is pretty easy. Once you've had enough of your iDisk, you can:
Drag the iDisk icon to the Trash.
Click on the iDisk's icon and hit -E; this is the same as selecting File Eject from within the Finder.
Open a Finder window, hit Shift- -C (for "Computer"), select the iDisk, and hit -E; again, this ejects the iDisk from your system.
Unlike other mounted disks, the iDisk does not have an eject icon next to it in the Finder's Sidebar when it is mounted. This is unfortunate, because when you're looking at a Finder window, there really isn't a way to know that your iDisk is mounted unless you physically click on its icon (and even then, if your iDisk isn't mounted, it soon will be if you're connected to the Internet).