stopping spam from own domain

[pal at hkskole.no]

Running Mailscanner version 4.44.6 and latest version of sendmail on
Fedora core 3, I have a problem with a lot of spam sent to my domain. The
spam mail are recognized as sent from my own domain, with fake sender and
fake receiver addresses.

My domain is example.com, and the mail are sent to george at example.com from
admin at example.com. This is of cource not true.

How can I get rid of these mails?
--
Pål Monstad