If the brain does not take part in reflex action so how is our body forced to move our hand away whenever we touch a hot thing ?

Simple reflexes are controlled by the spinal cord. The message travels from the sense receptors near the skin through the afferent nerve fibers to the spinal cord. In the spinal cord, the messages are relayed through association neurons to the efferent nerve fibers, which carry them to the muscle cells that cause the reflex movement such as moving away of our hand.