Need help blocking spam by registrar

[vince]

I am receiving spam from an ever changing list of client IP addresses.  Each 
spam has 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 
a prescreen or SMTP time lookup of the registrar/owner of the client IP 
address to REJECT the connection?  Is this a workable solution?  I've 
searched the web, the documentation and archives.  Help.  (-: