So I guessed right, you were using the System Monitor. There are two because one is from Gnome, the other is from KDE. Both have different purpose and are a little different from each other. KDE-one gives you more control.
To kill a system process sometime you have to be root. Ordinary user-level control won't do.
You are absolutely right in principle about CLI.
su is used to be root in a terminal. ksysguard is the KDE-one
You could use it, or use "kill -9" with su. For that you need the PID. Search anywhere, not necessarily Google. You will get answers.
I don't like to offer everything on a platter. I would then be robbing you of the pleasure of self-discovery. I have given you enough hint.
Regards.
And you are welcome!