Rhubarb tends to start flowering fairly early as the leaf stalks are growing out. The flower comes out of the plant on a central stalk that usually has a couple of leaves attached to it. The best way to remove the flower stalk is to pull the stalk out of the plant rather than cutting it off. That also applies to harvesting the leaf stalks. It's better for the plant.
If you leave the flower stalk on the plant to develop seeds, it will usually cause the plant to stop growing more leaves, so you will get a better harvest of rhubarb if you remove the seed stalks .