The title says it all.
In olden times, one could right click the bar, go to "add to panel", and then add the "force quit" applet from the list. Apparently that doesn't work any more.
You can install indicator-forceclose. There is an article on it here. The home-page is launchpad.net/indicator-forceclose.
To install, extract the archive,
You can also add a similar program to the unity launcher, which lets you freeze/unfreeze programs as well as kill them. See this page for details.