Since the rotation of the earth makes it more flat,due to the centrifugal forces, is it not possible for the force to completely flatten out earth into a plane ?
No, because the Earth is currently (approximately) in a hydrostatic equilibrium. There would be tendency for it to flatten if the rotation speed became faster, but since exactly the opposite is happening, the Earth would likely become (even) more spherical in the future.
The centrifugal force from the rotation will indeed try to drive the Earth towards becoming a plane. However, this is balanced by gravitational attraction (or surface tension in the case of small spinning liquids) which attempts to make the Earth spherical. The final shape will be a balance between the two, which is an oblate spheroid.