A Stoic answers
How would a Stoic deal with anger?
A Stoic sees anger not as something that happens to you but as something you do: a judgement that you have been wronged, plus the decision to lash out. Because it begins as a judgement, it can be refused. The Stoic puts a gap between the provocation and the reaction, questions the story they are telling themselves, and remembers that other people act from their own ignorance and limits, not to injure you personally.
The principleAnger is a kind of temporary madness that costs more than the injury it answers. Seneca called it the most hideous and frenzied of the passions, and argued it is never useful, only sometimes excusable.
What the philosophers say

Marcus reminded himself each morning that he would meet the ungrateful, the arrogant and the dishonest, because they do not know the difference between good and evil. Expecting them, he was not ambushed by them, and their faults could not become his.

Seneca wrote a whole book on anger. His advice: the best remedy is delay. Give the first heat time to fall. Ask whether the offence is as large as it feels, and whether the person meant it, or simply did not know better.

Epictetus would point out that it is not the person who insults you who wrongs you, but your own opinion that you have been insulted. Change the opinion and the injury disappears.
Try this
- Delay. Do nothing while the first heat is on you; anger always promises that waiting will cost you, and it always lies.
- Question the impression: is this as bad as it feels right now, and did they truly intend it?
- Assume ignorance, not malice. Most people act from their own limits, not against you.
- Picture the version of you that stayed calm. That is the one you will respect tomorrow.
Questions
Did the Stoics think anger was ever useful?
No. Seneca argued in On Anger that it is never useful, only sometimes excusable, because it clouds judgement and does more damage than the wrong it answers.
What is the fastest Stoic technique for anger?
Delay. Put time and space between the provocation and your response, and let the first heat fall before you decide anything.
Ask them yourself
Put your own situation to Marcus, Seneca or Epictetus and hear them answer in their own words.
Talk to a Stoic