The way I do it is oen a terminal window,

> ps ax | grep <process name>

where <process name> is the process or programme you want to kill

> kill -9 PID

where PID is the Process-ID of the process/programme in question (you
get this from the ps|grep command above)

oh, and ">" refers to the command line prompt

There may be a more "graphical" way of doing this, but I've never
looked for one.