Anger is generally a reflective of what Daniel Kahneman refers to as Type 1 thinking/processing (
Thinking, Fast and Slow - Wikipedia ). That is the near reflexive, largely unconscious, processing that occurs within the first instants of some information being presented. We do a superficial threat-assessment, and react accordingly. As much research has shown, it is VERY hard to suppress, though it is equally clearly a product of learning, and so can be unlearned (or at least replaced by other learning). As such, I think it would be hard for anyone to avoid reacting in irritable manners if the associations that prompt it are deeply entrenched. Simply "making up your mind" will have little power. Moreover, automatic hostile or mistrusting responses will tend to provoke similar in others, which too often serves to further entrench the Type 1 reactions. If people are prodded into behaving like outright pricks, then it becomes hard to escape the automatic and implicit association that they ARE, and will behave like, pricks.
HOWEVER...
Willpower CAN be productively incorporated into Type/System 2 thinking. It may be difficult to suppress initial automatic responses, but there is nothing, except for maybe willingness and usable "scripts", that would prevent one from apologizing or otherwise defusing situations when one's initial reaction was unnecessarily hostile.
"Maybe that came off a little strong"
"I'm sorry, I wasn't angry at you in particular"
"I hope that didn't come off mistrusting or hostile"
etc., etc.
With time and practice, the consequences of such Type 2 thinking, and the resulting interactions, can come to replace the more conflict-prone Type 1 processing.