@so lucky I'm a seed saver, I grow beans, beans and more beans, in a small back yard garden. I really haven't had a problem with crossing so far but I usually bag several flowering stems just when they're starting to form, you can make your own bags or what I have done of late is buy wedding favor bags...
These can be reused over and over again, notice I have turned the top edge down on the white one so it's easier to snug it right in against the bean vine. The only thing is with these is to make sure you remove them once you see the beans starting to form. If forgotten the beans will curl up in a ball and they're hard to remove. When removing the bags I tie a piece of green garden tie tape around these ones so I know not to pick them.
Of course there are a few varieties of P. vulgaris that are notorious for crossing like Ma Williams, when growing these I try to give them as much isolation as possible usually out front, with this one I bag a lot of flowers and only keep these ones for seed, we eat the rest. The seed from the bagged flowers I grow are used only if or when I think I have a cross or if I have enough these are the ones I use in trades. So far the crosses that have occurred in my garden to date have been from seed I've gotten in trade.
One exception though, this variety 'Mr. Tung's' a bean that came to Canada over 100 years ago from China, it's a tender good tasting snap bean.
Years ago I had one bean in a pod that was a different color than the rest...

The brown ones are what the seed is supposed to look like, I planted this pink seed and got pods like this, totally different in size and shape...

.......................................................................Shelly and dried seed from these pods.
The next year I planted one of these and got this...

Big fat white seed with pink speckles and the pods were heavily marked in rosy red. Still have this seed somewhere but haven't grown it again to see what it would produce. By the way the pods were tough and fibrous, not worth eating.
Runners are notorious for crossing so I only grow one runner each year, my neighbor also grows a runner but she very kindly grows the same one I do each year, I supply her with seed

, still I always bag one or two flowers as a safe guard as you can see in the above picture.
Annette