Here is my patented spam blacklist:

i=0;o(a){printf("%u",i>>8*a&255);if(a){printf(".");o(--a);}}main(){do{o(3);puts("");}while(++i);}

Compile with gcc and:

./a.out | xargs -L 1 -I target iptables -I INPUT 0 target -j drop

Edit: Wouldn't want to piss off the BSD users, so:

./a.out | xargs -L 1 -I target ipfw add 0 deny ip from target to any

Soon you will find that any and all spam problems have permanently ceased!