Denying e-mail based on a word in the body of message --> in sendmail 8.11.6

If an e-mail is addressed to one of your group members and it contains the
word "viagra" within the
body or subject of the message. How can you block receiving this type
messages and send a message back
to the sender; informing them, "to cut the sh.." as a warning.

Please reply.  Thanks Kevin