You are missing something on your hack correction process.
If the hack is coming back, you have still malware in your site (called a rootkit or backdoor). I use both iThemes Security and WordFence and both together can find more hacks than just WordFence alone. Try to look for backdoors as they allow for re-hacks.
This document from Securi explains the DarkLeech hack:
https://blog.sucuri.net/2015/03/pseudo-darkleech-server-root-infection.html
Securi has tools to look for this hack.