They say
“That’s just how I am.”
Your response
“I’m not asking you to change who you are. I’m asking you to change a specific behavior because of its effect at work. What would a workable change look like for you?”
First, check this
Have you described a specific, changeable behavior and its work impact, or are you criticizing a personality trait or communication style?
Why this works
The employee has turned a request about behavior into a claim about identity. Separating the two makes clear that adapting conduct is not the same as becoming a different person. Asking what a workable change looks like gives the employee some responsibility for finding a genuine alternative. This page must stay with observable behavior and must not diagnose personality, attitude, or character.
Don’t say
“Then who you are is the problem.”