Once a banana tree has fruited, the tree needs to be cut down. The flower arises out of the center of the tree on the growing tip of the plant. Once it's made a stalk, the tip stops growing. So, you chop the tree down.
Some people chop the trees down to the ground. Others, like me, prefer to leave a stump that's a couple of feet above ground. The sap and water that is in the stump flows back down into the roots making the baby plants that come up around the stump healthier.