There are a number of ways one can forcefully quite a troublesome application in Ubuntu, including using the system monitor. A while back there was an app-indicator applet, which allowed performing this task with convenience. Unfortunately, this applet is no longer compatible with Ubuntu 11.10 Oneiric Ocelot. Moreover, Gnome 3 has modified the way applications are exited forcefully. Recently, a third-party script has been developed to integrate Force Quit functionality to the Unity Launcher. Instructions for installing this script are given below.
You can download Force Quit script from the direct link at the end of this post. After downloading the Force Quit package, extract the compressed folder. From the extracted archive, right-click on the Install.sh file and extract it. Once done, execute this Install.sh file and select Run from the dialog box that pops-up.
Once done, you will be asked to enter your password, after which Force Quit script will be installed.
When installation is complete, you will be provided with a dialog box to open a folder containing numerous shortcuts. Click OK to open the folder and locate the Force Quit shortcut.
When found, drag and drop the Force Quit (script) applet to integrate it with the Unity Launcher. This will allow you to forcefully quit applications from the Unity Launcher.
You can download this script from the direct download link given below.
Download Force Quit (Direct Download Link)