[Nek5000-users] how to turn off conductivity/diffusion

I would like to model scalar transport without diffusion. Now I model scalar transport with the help of temperature variable (as in examples.pdf). If I just set parameter p0008 to zero or less than e-7 I got exception (background scalar gradient is about 100), which is quite comprehensible because of the insulated boundary condition. Even e-7 is quite a lot for my problem. So is there a way to model scalar transport without diffusion, i.e. without RHS at all? Or may be some trick with boundary conditions, which allows to set diffusion coefficient to zero?