The entire documentary made me very sad because I could not think of any solution
AFAIK that's not an accurate depiction of what monsanto has been doing. According to wikipedia[1] monsanto says they won't "exercise its patent rights where trace amounts of our patented seed or traits are present in farmer's fields as a result of inadvertent means". I'm not sure what the exact that was discussed in the documentary, but the ones mentioned in the wikipedia article all have elements of the farmer doing something intentionally, eg.
>The case began in 2007, when Monsanto sued Indiana farmer Vernon Hugh Bowman who in 1999 bought seed for his second planting from a grain elevator – the same elevator to which he and others sold their transgenic crops.[17] The elevator sold the soybeans as commodities, not as seeds for planting.[17][18] Bowman tested the new seeds, and found that ,as he had expected, some were resistant to glyphosate. He intentionally replanted his harvest of GM seeds in subsequent years, supplementing them with more soybeans he bought at the elevator. [...]
[1] https://en.wikipedia.org/wiki/Monsanto_legal_cases#As_plaint...
Well that's telling, in that they still claim to have the right to shut down such a farmer.