The license used is:
"It basically says these seeds are free to use in any way you want. They can't be legally protected. Enjoy them."
This is a GPL type license. There is nothing to stop Monsanto from going to a farmer who is using these seeds and saying:
Pollen from one of our products blew in last year and so these seeds now contain some of our genes, so you now owe us for using these seeds and can't give it away to anyone.
The only way to deal with Monsanto is to beat them at their own game. One way would be to develop a seed with some novel genes (call them NoGe) and copyright these under something like the GPL. Then grow these seeds upwind of a Monsanto development facility; when, later, Monsanto then sue someone for illegal use of their seeds a NoGe 'owner' could testify that the Monsanto seeds must be allowed free to everyone use due to the 'viral nature' of the GPL. That legal punch up would be interesting to watch!