What kind of spoofing are you talking about? If it is e-mail spoofing, that is really impossible to prevent since SMTP is not an authenticated protocol and the headers can easily be manipulated to forge the from address. Think about the return address on an envelope. Anything can be written on that area and yet the message will still get delivered to the recipient.
Another way of looking at this is how backscatter happens. This is discussed in this <a href=”http://itknowledgeexchange.techtarget.com/itanswers/how-to-find-and-remove-bots-from-your-network/”>question/answer</a>.