Since it is the nodules in the roots that "fixes" the nitrogen, it is either done or not. Pull up a plant and look at the roots. You will see little teeny tiney "potatoes" on the roots. Those are what fixes the nitrogen.
If you have a bug infestation. I would get rid of the surface plant, either in a hot compost pile, or the chickens. The chickens will love the bugs and the greens.
I think nipping the bug infestation in the bud/butt is more important than fixing nitrogen.
Optimally, if you want to fix nitrogen, you would plow the bean under, but even without plowing the plant under, you will be ahead of the game for next year! For fixing nitrogen you should plant beans in the same spot next year, they should do better. I don't know how that works with the specific bug infestation you have. If the bugs over winters in the ground, you may have to start over elsewhere. I would also let chickens in that part of the garden to help erradicate the nasty little bug.