I can understand that this issue might be causing you a lot of distress.
But here, firstly it is improtant to find out from where your child picked up this gesture, because at this age children mostly learn by observing and immitation. So find out is it from a cartoon, or any other kid in the school or neighborhood, etc. And try to have conversations about it as well with your child- that from from where he picked it up, what incidences or emotions act as a trigger for him to react in this particular way. And then slowly try to model other ways of showing anger by doing that yourself in your daily life, i.e. by talking about the emotion instead of acting, being quiet for a while and then come back when you feel okay, etc.
And now secondly, for children, what works more effectively to give them an understanding of right and wrong, is the consequence of their action. So when they get praised or rewarded, they feel that is a good thing to do and they repeat it. But on the other hand, just mere scolding might just indicate an absence of consequence to them, right? Therefore when they are doing anything wrong, that too requires consequences like maybe taking out their TV time or play time, or maybe increasing 2 more pages of home work or 30 more minutes of study, or something like that.
So try to understand why is he choosing this mode of expression, and then explain why it is wrong- storytelling using morals can be an excellent medium, to give them a simple understanding. And then try to maintain some strict ground rules at home, breaking of which might come with small harmless consequences. Hope with time, your child too, will be able to differentiate..
Hope this helps!