There isn't an option to purge o delete a package in the preseed configuration script but you can use this command....

d-i preseed/late_command

This command is run just before the install finishes, but when there is

still a usable /target directory. You can chroot to /target and use it

directly, or use the apt-install and in-target commands to easily install

packages and run commands in the target system.

"in-target" means: chroot /target

d-i preseed/late_command string [in-target] foo

example :

d-i preseed/late_command string 

            in-target apt-get remove packagename

you can also run a script :

d-i preseed/late_command string 

        in-target wget http://........./postinst.sh -O /root/postinst.sh; 

        in-target /bin/bash /root/postinst.sh

or install a group of DEB files :

d-i preseed/late_command               string 

    for deb in /hd-media/*.deb; do cp $deb /target/tmp; 

    chroot /target dpkg -i /tmp/$(basename $deb); done