Killing apps from the command line will just axe threads one at a time. Since the BeOS is aggressively multithreaded, you might have to do a lot of killing before the app finally dies.
Easier is the „Vulcan Death Grip“ Tracker technique. Hold down the right Cmd,opt,ctrl, and shift keys and then click on the App’s icon in the DeskBar. The offending app should die a swift death.
[Editor’s note: On PC keyboard, use Ctrl+Alt+Shift.]