This is a variation of the bat book‘s subject header checking trick. Assuming that you want to block messages based on the content of the Subject: header of an incoming message, you can place the following rules into your .mc file:

LOCAL_CONFIG
HSubject: $>BlockSubject

The above basically instructs sendmail to call ruleset BlockSubject with the value of the subject. On with the ruleset now:

LOCAL_RULESETS
SBlockSubject
# The next rule is broken in two for readability!
R$* test - block this message $*
        $#error $: "553 message blocked due to Subject: " $&{currHeader}
R$* Your new password $*        $#discard $: discard
R$* Casinoo $*        $#discard $: discard
R$*        $#OK

(You may want to change the $> operator with $>+. Read paragraph 25.5.1 of the fourth edition of the bat book for a discussion on the matter.)

The bat book prefers to put all the unacceptable subjects in an external database file (which is maintained much like aliases and virtusertable). I prefer keeping the list of the unacceptable subjects inside the .mc file for two reasons:

First, keeping them in a file outside the .mc makes the list grow faster. Editing the .mc to add yet another unacceptable subject makes one think whether to do so or not.

Second, although a subject that contains a certain phrase may be considered unacceptable, you might want to make an exception. For example one may decide to block all the Your new password messages except ISP name – Your new password message that your MIS sends to your users when they reset their password. This can easily be maintained in one place in the .mc file and is also self documented modem noise code:

R$* ISP - Your new password $*        $#OK
R$* Your new password $*        $#discard $: discard

Remember, do not copy-paste sendmail.mc code. The LHS and the RHS are tab separated. Copy-pasting converts tabs to spaces and your ruleset will not work.