Experts and BHG readers answer.
Harvesting rhubarb gone to seed
You should break off that seed stalk. And, if you have any harvestable sized stalks you can still use them, BUT it's getting to the end of the season so I might let the plant go another year to build up its strength. You don't say how old your plant is, but if it's a young one, I'd leave it alone now. Mid June is about the end of the season anyway.