Need help blocking spam by IP address owner

[vince]

I am receiving spam from ever changing client IP addresses.  Each spam has 
other tiny variations that prevent string matching.  The one thing they all 
have in common is the owner of the IP addresses.  Is there any way to do 
lookup of the client IP owner at Prescreen or SMTP time to REJECT the 
incoming connection?  Is this a workable solution?  I've searched the web, 
the documentation and archives.  No luck.  Does anyone have a solution or 
script?  Thanks!