SPACE. Lots of space. Or lots of greenhouses. A physical barrier, or an insurmountable distance are the only way to keep pollinators seperated. and of course greenhouses have their own problems to overcome - particularly if the plant relies on an insect pollinator for reproduction.
Be warned - this post is a lot of stream of conscious here...Hm, I have enough space for corn, I think. Unfortunately, the neighbor windward of me grows corn. Or I could bag the ears... harder than bagging the tassels but I doubt the neighbor would appreciate me bagging the tassels of his corn and I don't have time to bag that many acres of corn. Well, learn something everyday - it is possible to bag small grains also... researchers did it to measure pollen production. Not the least bit practical, I'm sure, even for just 5 hens, not even counting what we would eat directly, if I wanted to be consistent in doing this.
Buckwheat grows well here and is insect pollinated.
But does this matter? As long as none of the neighbors have modified crops that can cross pollinate with what I'm growing - it shouldn't. Corn, Alfalfa, and squash are on the list of GMO foods grown in the US; not the rest of crops that might work for this purpose in my climate. Edit to add potatoes are also but pollination isn't a problem with them.
Sweet corn needs to be planted more than 250' from field corn or pop corn for it to reliably have sweet corn characteristics but this
https://enveurope.springeropen.com/articles/10.1186/s12302-014-0024-3 says, "...Maize pollen deposition was negatively correlated with distance from the nearest pollen source. The highest pollen deposition was within the field, but depositions of several thousand pollen grains per square meter were recorded over the kilometer range...."
They said wind direction and terrain matter. They didn't say how big their original fields were. It makes sense that a small field of corn (likely in Europe? maybe? at least compared to me here in the midwest) would disperse a lot fewer grains of pollen per square meter than the multiple 40 acre fields upwind of me will.
Alfalfa and squash are bee pollinated. Honey bees forage an average of 1 to 2 miles. Between the squash fields, the bean fields, and the orchards, there are a LOT of apiaries within a couple of miles. Maybe with so much bee pasture, they would forage on the lower end of that range of distance? Maybe. Squash wouldn't matter as much since it is pretty deliberately planted each year - I could bag the female flowers and hand pollinate them to get enough seed for the next year. Alfalfa would be much bigger problem - it is far too expensive to plant such a perennial as if it were an annual and it reseeds itself at least partially. I could probably find all the alfalfa fields within a couple of miles and ask if they are gmo varieties... advantages of knowing the neighbors.
I mind less that gmo is being done somewhere as I mind that it is so hard to avoid that being the only option.