Try going to
www.dnsstuff.com and get more information about the ipaddress.
also find out if the servers are blacklisted.
You will need to block ip ranges which you can get on entering an ip in dnsstuff.com.
To get an idea of ip range to block check this out.
http://jodies.de/ipcalc
Now depends on how ruthless you want to be. If you start blocking ip ranges soon you will have them all blocked out but quiet a few innoncent chaps as well so it is a balancing act.
also block free proxy servers.
should get more info here
http://www.de.sorbs.net/