Removing dock icons in Mountain Lion
To remove an icon from the Apple dock, just click and hold the icon, drag it off, release your mouse, and poof it disappears. If you’ve upgraded to Mountain Lion (10.8.2) you may notice that this doesn’t seem to work. Apple didn’t disable this feature, they just changed it so you have to hold the icon a few seconds longer before it disappears. Just click, drag and hold the icon off of the dock for at least 10 seconds till the poof cloud appears and then release.