The javascript solution has already been suggested, but take a step back and think about it: the same way the leech worked out your links, domain name, logo and all the stuff that brands your website, he can easily figure out the simple JS code suggested here.

<img src="x" onerror= "if(document.location.href==='http://4co.in')document.location='//xxxxxx.xxxx';">

So I say, go a step further:

- do not send his users to a black hole, instead show a banner warning them about the leech and then after a few seconds redirect the user to your website.

- The JS code for the above should go in the same JS file that provides core functionality to your website.

After done that, run your JS past http://closure-compiler.appspot.com/home or if you better still install the yuicompressor cli (http://yui.github.io/yuicompressor/) in your machine.

The resulting code will be minified/compressed and seriously obfuscated. So trying to defeat it will that the leech hours if not days depending on his experience.

- encode/obfuscate the warning string (1st topic) to make it harder to find within the JS code.

- and finally do a daily spot check on website following @jarrett comment below