Hi Bill, this is really something that is set within the Operating system being used to control the homepage of the browser. NT/@k/XP offer administrative tools built in to disallow changing the IE homepage. 9x has the Policy Editor which can do it.
This is really something that cannot be acheived by anything within your own site (intranet/internet).
Actually that's not quite right, but since I know of the havoc that homepage hijacking can cause I think it is irresponsible to publish the technique on a respectable public forum.
I really think this is something that needs to be set and enforced within the operating environment.