How to I keep one person from commenting on my FB site?

You can't stop people from commenting on your posts... Unless you hide the posts from them.
To do this, every time you go to write a post, click on the option next to 'post' where it lets you choose who to share it with. There will be the options 'public', 'friends', and 'friends except'. Click on 'friends except' and then select the person's name.

If you're that irritated on how that person comments, why keep him/her as a friend? Just for the sake of having many "friends"?

Add them to your Restricted list on your Posts Privacy Settings

that's not possible - you would have to add them to your restricted list, and then they wont even see your posts

http://abrax.us/Facebook/RestrictedList.php