CBOT says wheat is deliverable if it contains up to 4ppm. That doesn't necessarily imply that all CBOT wheat == 4ppm vomitoxin. The answer though, would be to blend it with more expensive wheat, or to clean it with sorting machines, which they do anyway.

Edit: Here, sorting machines https://encrypted.google.com/search?tbm=isch&q=optical%20sor...

It mentions in the end in the footnotes you can mix together high vomitoxin and low vomitoxin wheat to produce the needed amount.

  I'm kidding. My dog is gluten-free. Also if you buy 
  the high-vomitoxin wheat you can blend it with 
  low-vomitoxin wheat until you get the perfect amount 
  of vomitoxin. Appetizing, no?

You can mix it with more pure flour. But I thought last was just reselling the 4ppm flour not using it themselves