My first piece of advice would be to appeal to their human side - state your case that it's your hard work they're stealing and that duplicate content may actually hurt both of your rankings.
If that doesn't work which I expect it wont, dend a strongly worded Cease and Desist letter, you'll find some templates here:
http://minnesotaattorney.com/cease-desist-letter-template-example-sample-forms/
Ensure all of your content has a clear copyright mark.
Use the letter to make the point that legal action may be taken if they do not stop.
good luck!
