There are numerous ways of forcefully quitting an application in Ubuntu, including using the system monitor. Back in the day, we had an app-indicator applet which allowed performing this task with ease. With the advent of Ubuntu 11.10, this applet no longer works. Furthermore, Gnome 3 has modified the way applications are forcefully exited. Recently, a third-party developer has released a script to integrate Force Quit functionality to the Unity Launcher.
You can download Force Quit script from the link given at the end of this post. After downloading the package, extract the compressed archive. Within this archive you will find an Install.sh file; extract it via right-click context menu. Once done, execute the Install.sh file and select Run.
You will be asked to enter your password, after which Force Quit will be installed.
After installation you will be provided with the option to open and view a folder containing shortcuts; click OK to open the folder and locate the Force Quit shortcut.
Once found, drag and drop the Force Quit applet from the aforementioned folder to the Unity Launcher for integration. This will allow you to forcefully quit any misbehaving or resource-intensive applications with ease.
Since there is no home page for this application, we are providing you with a direct download link for this script.
Download Force Quit (Direct Link)