adding anti spam rules

[Gavin Nelmes-Crocker]

I'm struggling to work out how I can add a rule to block mails with specific
content in a mail

i.e. Want a free flowing and trouble free Septic Tank?

would I add this to spam.assassin.prefs? and in what way - the only examples
I've seen seem to be for info in the header,subject or from specific people.
What I need is somewhere or some method of adding specific sound bites from
within the mail - I realize this is risky if something was added that could
be legitimate but I'm assured no one wants the above or if they do they have
it already without assistance form some unknown person in another part of
the world tinkering with their septic tank.

Thanks

Gavin