How to remove a competing site?

This is really bad but if you want to get him punished you can start submitting his sites to already punished sites or to sites with inappropriate content. Google will follow this up and his site will drop in the rankings to a considerable amount. Maybe as much as 900 places. Now that is a thrashing.

Depending on what the hate page says... If it is slander and threatening in the slightest, then just get a standard letter drawn up from a solicitor/lawyer and have it presented to him.

Anything even slightly threatening and he is in big trouble.

I am sure you can find all the relevant info on where to send the letter to and it will probably be a lot more effective than going down the SEO path, because even though it sounds more expensive now, you guys playing cat-and-mouse with SEO against each other will probably end up being more expensive in the long run.