I didn't tried it yet, but gone do that tomorrow.

apt-get --purge remove kdelibs4 libarts1

that is what I found if you use kde 3.1 like knoppix 3.6 3.7

when you start your x desktop a file called .xinitrc i guess there is that you want to change somthing to make fluxbox the default desktop manager

I let you know somthing when I tried it