When you make your point don’t say anything more after that.
A few years ago I had a very challenging situation.  One of my employees had a very bad hygiene problem.  People would walk into our office and just about keel over from the odor.  It was throughout the office.  I had no choice but to talk to the ‘offending person—who was a she’ privately in my office. I delivered the news fairly tactfully and all went well.  She got up to leave and then for whatever reason I will not know,  I added “And I haven’t been able to breathe through my nose due to allergies yet I could still smell something all week:”  This brilliant addition ignited a torrent of tears flowing down her face. Aghhhhhh.  As a result, my tip is to “say what you need to and not one bit more.”