How to Remove Icons from the OS X Mountain Lion Dock
Removing an item from the Mac OS X Mountain Lion Dock is as easy as 1-2 (there is no 3): Just drag its icon out of the Dock, as shown in the top part of the figure, and it disappears with a cool poof animation, as shown in the bottom part of the figure.
Choosing Remove from Dock from the item’s Dock menu is another way to make the item go away.
You can’t remove the icon of a program that’s running from the Dock until you quit that program. Also, note that by moving an icon out of the Dock, you aren’t moving, deleting, or copying the item itself; you’re just removing its icon from the Dock.
The item is unchanged. The icon is sort of like a library catalog card: Just because you remove the card from the card catalog doesn’t mean that the book is gone from the library.
The Dock in earlier OS X releases included icons for the Documents and Applications folders; the Dock in Mountain Lion does not, at least not by default. Having those folders in the Dock is convenient, and you should consider adding them to your Dock if they aren’t already there.
If you upgraded to Mountain Lion from Lion, on the other hand, the Documents and Applications folders will appear in your Mountain Lion Dock, unless you removed them prior to upgrading.