Adults who need constant validation are like needy children who have not fully developed into maturity. In this case, they need understanding.
However, when one needy individual crosses the line to bear false witness against another just to feel good about him/herself or for whatever skewed motivation, then this must not be condoned by matured adults who know better. Bearing false witness on another human is a grave offense in any moral code, even in the Judeo-Christian tradition. This act violates the rights of another human to dignity among peers and the community at large. This must never be condoned otherwise, when good people stay passive and tolerant of such wrongdoings, they do irreparable damage to the needy person.
The best way to discipline them is through show and tell. It is never our place to smite them with "lessons." We are not God or gods. Meaningful realizations are made through self-examination and discovery rather than outright sermons on the wrongness of their thoughts and actions. Consider it "tough love" and we will be doing them a favor as we want to help them grow into well-rounded, well-adjusted and adaptable individuals among peers and in fellowship with humankind in the community of humans.