Avoiding updates in preseeded install

I have a working preseeded install that boots off of a network using PXE. The install is entirely interaction free, and boots me up to a nice, functional 12.04LTS system. However, there is a particular update I wish to avoid. I can't seem to find any way of either disabling the precise-updates channel, or specifying that particular packages be frozen at a particular version in order to ensure that the system comes up in the state that I want. Is it possible to do this in a relatively simple way, or is it unsupported?

Thanks,
JJar