Do you have problems with spammers? Would you like to just slam the door on them? If you have sendmail, here's how (requires support for long macro names, and for the ruleset check_mail (I know that sendmail 8.8.0 and on supports this)):
# Spammers Galore!
F{Spammers} /usr/local/mail/spammers
LOCAL_RULESETS
Scheck_mail
# Bounce junkmail sites
## This one covers complete addresses given in the Spammers file
R<$={Spammers}> $#error $@ 5.7.1 $: "553 Spammers Galore! We don't accept junk mail."
## These two covers host or domain names given in the Spammers file
R<$* @ $={Spammers}> $#error $@ 5.7.1 $: "553 Spammers Galore! We don't accept junk mail."
R<$* @ $* . $={Spammers}> $#error $@ 5.7.1 $: "553 Spammers Galore! We don't accept junk mail."
## And now, to the normal parsing
R$* $: $>3 $1
## Did it become a spammer? GALORE!
R$* < @ $={Spammers} . > $* $#error $@ 5.7.1 $: "553 Spammers Galore! We don't accept junk mail."
R$* < @ $* . $={Spammers} . > $* $#error $@ 5.7.1 $: "553 Spammers Galore! We don't accept junk mail."
You have to understand the code above. Make sure you place the tabs in the right places!
I'm not responsible for the upcoming of that hack, but I've enhanced it a little.
And oh, in the spammers file, you can have domain names or complete mail addresses, one per line. Here's an example.