Debian Preseeding: prevent particular package from being installed

[Konrad Vrba <konrad.vrba@gmail.com>]

I am using Debian preseeding for automated installations (from PXE)

in my preseed config file, I have the following line, to install only
the minimum number of packages:
  tasksel tasksel/first multiselect
  d-i base-installer/install-recommends boolean false

This however still installs some packages which I don't want. My question is:
  how does the installation script decide which packages will be installed?
  how can I modify this list?
  can I prevent specific package to be installed

or, is there some other Debian mailing list, which would be more
appropriate for my question?