to avoid spam i want block mail for which Return-Path: and From: are not same.how can i do that?

because we are getting spam mail for our our users with sent address of our domain but with different Return-Path:
so i want block such kind of mails....

another is there any way to block mail which having perticular word in there subject or body ...for all users.
for example i want all mail which containing word "******" in there subject or body part for my all users.