emo·tion·less | \ i-ˈmō-shən-ləs \
Definition of emotionless
: showing, having, or expressing no emotion :an emotionless stare
Other Words from emotion
Is an emotionless reaction really a reaction? If the intent is to agitate or unnerve, then the expected result is absent. Yet, there was written or oral words occurring as a result, so it would have to be a reaction.
In an act of non-cooperation with individuals that seek to agitate it is best to provide only emotionless responses. Perhaps even to the point of indicating a lack of understanding for the purposes of blunting their efforts.
So much stress can occur when emotions are applied to responses. This may be what the perpetrator seeks. There is no reason to provide that avenue. It is suggested therefore to reject knee-jerk reactions that come to mind and wait, calming yourself, and preparing a emotionless reaction instead. This may indeed tend to un-nerve those that seek to un-nerve you.
This response would not be an indifferent response.