Since Monsanto claims ownership of any plants that happen to grow on a farm that when tested shows their patent. I think it would be wise for farmers who do not use any Monsato products to take the offensive and sue Monsanto for trespassing, contamination and grand theft.
Trespassing - Since none of their products were used how did it get there? Who gave access to these Genetically Modified seeds to their farms. Since the farmers didn't then Monsanto is responsible for taming or controlling their product.
Contamination - The farmers crops are now contaminated with chemicals that they did not use or authorize and as such has destroyed the integrity of their farm.
Grand Theft - Since the law states that any products found with Monsanto now belong to Monsanto, the farmer could argue that since he/she didn't farm with products Monsanto and Monsanto products mysteriously appeared on their farm, Monsanto has stolen the farmers plants as a result of their trespassing and contamination.
The farmer would then sue for the cost of removal of Monsanto products from the farm, lost revenue as a result of this Monsanto takeover and other damages. I'm not a lawyer but this would definitely be something to think about and sounds like good grounds for a class action suit or just one lawyer with a lot of resources.
Even if the farmer doesn't win, it would create conversation and perhaps a reversal of the current law regarding Monsanto products.