Next year just break or cut those flower stalks off as they appear. The plant doesn't need to expend energy producing flowers and seed - well, not for your purposes anyway.
We have gone over our 3 rhubarb plants once, harvesting all the leaves. They will come back and more will be available for harvest. After that, we'd better leave them alone altho' we have harvested a little rhubarb in the fall.
The best rhubarb plants I know of, including the 3 plants here in the yard, grow on the east side of a building. They just seem to do real well in that setting.