I'm quite surprised that so many people are saying there's nothing you can do.
Write the letter and send it to him and his abuse department, if any of the articles have been taken off an article directory write to them and tell them they're expressely breaking the terms and conditions.
Also investigate the rest of his content, if he's doing it to you, chances are he's doing it to other people, contact them and inform them.
Take a look at his host, investigate the terms and conditions and inform them of infractions.
Something else you can do is start writing the articles in HTML, throw in a few java script extras and start scrambling your code. This wont stop anyone that determined, but a lamer who's resorting to stealing articles isn't going to be able to work it out.
Another option would be taking a note from Flikrs book and adding a transparant image over your articles for a few days until they're indexed.
All off thats of the top of my head and wouldn't be that difficult, I'm sure there's plenty more that you could do