Ban or Block .ru from server

Some one from .ru keeps injecting a script on my index.php pages.
I don't know how they are getting in.
I have totally stopped ftp.
They keep injecting it.

It is obfuscated script, but I changed it to alert() so you can see it here:

This is the code injected:

I want to block everything from .ru from my server. Is that possible?
How do I tell how they are getting in?

Closed (fixed)