Can I keep users from receiving mail from the Internet?

[Fedora]

I'm using sendmail as the MTA and I would like to make it impossible for
certain users (root comes to mind, but there are other userids as well) to
receive email from the Internet.

I'd like to "block" it, as opposed to having some sort of filter throw it
away after it's arrived.  Ideally, these few userids would appear to be
invalid to anyone outside my network.

On the other hand ... I do need for all users to be able to receive email
"internally" (i.e., from localhost).

In my case, spammers have discovered that there is <gasp!> a "root" user
that they can send email to.

Any thoughts?

Thanks much,
Charlie