Use the following method to accomplish this. However, I must warn you not to use this method for normal filtering as messages will get rejected without any trace and you will have no way to know what happened.

Steps:

• Create a file called CustomRejection.dat in $INSTALL_DIR\config folder
• Every line in this file will contain one rule separated by three $ signs.
• RHS of the $ signs should contain the sender, LHS should contain recipient
• The word ALL signifies everyone.
• Lines starting with # signs are ignored and can be used for comments

Sample file

# This is a comment
# The following example shows how to reject every email from bad.boy@somewhere.com to 
# every recipient in your company

bad.boy@somewhere.com$$$ALL