N1K

Although not cryptolocker full protection it should help. http://www.wincert.net/security/cryptolocker-virus-protection/

I find that hard to believe as long as it brings you income and I believe it still generates decent earnings You can easily sell this site for a nice amount rather than shut it down, so I don't see the point of your statement.

Check for his dynamic IP changes history and ban the whole range of IP addresses. In this way, you will probably ban potential innocent visitor, but you will get rid of this problematic user.