At this stage (flowering), the plant will be harder to control. In the spring before flowering is the proper time to control with herbicides. 2,4-D three way mixes as well as glyphosate work well on non-flowering plants.
2,4-D is selective and will control broadleaf plants only. Glyphosate is non-selective and will damage any plant it touches.
Mowing before flowering can do a pretty decent job of control and will decrease the amount of seeds produced. Be aware that this is a poisonous plant. Getting the sap into your eyes or mouth is very dangerous.
Poison hemlock is a biennial, it germinates and produces a rosette the first year. The second year it sends up flowers.
Hope this helps.