I have the same problem:

Whatever I do, I dont need a password to be root; I only need to type "su".

I tried:

chmod 0 su

but it blocks at start

or

I tried to disable knoppix in /etc/sudoers

but then some applications doesn't work because they need root.

I found a half-solution:

I modified /etc/profile. Icommented this line (the last one in the file):
#alias su="sudo su"

Now if I type "su", I need a password.

But if I type "sudo su", I don't need it anymore.