If a program stops working or freezes, you can force it to close ("kill" it) using System Monitor. Go to the Processes tab and click to select the process you want to kill. Then, click End Process.

If the process does not close after a few seconds, right-click it and select Kill from the menu that appears.

Using End Process tries to close the process properly, giving it time to save files and so on, whereas Kill forces it to close straight away. For some programs, this might mean that you lose unsaved files. You should always try End Process first.