Some modern branches of Reiki claim that a Reiki practitioner must protect him or herself against the energy of others with certain techniques and practices. These are often developed with practices taken from the New Age movement.
It is true that one can affect others with energy and if someone wishes to do harm energetically, then it might be possible. This is commonly called a psychic attack.
Traditionally, it is taught that to protect oneself from outside influences, one must be strong within. Therefore the most effective protection that the system of Reiki could offer a practitioner would be the ongoing practice of a technique like hatsurei hô. With this technique the practitioner’s original energy strengthens, and offers a deeper awareness of energy. In this way the practitioner can sense what is happening and yet is not necessarily affected by it.