Need a virtual host setting up in WHM/ apache

Someone is pointing their domain at our IP. I need a virtual host setting up that will redirect their traffic to a 404 page on our server, without affecting legitimate traffic to our site.

See here for a solution I found which describes our problem
https://stackoverflow.com/questions/25236810/how-do-i-block-someone-elses-domain-pointing-to-my-apache-hosted-website

We have already had a run at this problem and found that the expert we chose was unable to block the traffic. Please give a rough outline of your approach when you bid.