I have dealt with 2 children (one my own dd, the other a sibling) who have had violent meltdowns post trauma. And neither of them had actually witnessed violence.
To me, punishing for something so completely out of control didn't seem any more logical than punishing dd when she lashed out during delirium in a high fever. Later conversations with dd confirmed that she hardly knew what she was doing during one of her meltdowns so in my view quite similar to a fever. I don't think my failure to punish her when she had a kidney infection confirmed that it was ok to have a kidney infection. PTSD is like that. It's an illness.
A punishment, apart from being (in my view at least) unjust is also counter-productive as serves to prolong the memory of the stress, give the child the idea that that is who they are. Both my dd and my db needed the respite of the periods when this was not who they were, when there was time for them to laugh and be happy and feel good about themselves. Our motto was always "do not let the sun go down on your wrath" and that worked well.
Both my mother and I followed very similar procedures and as we have produced two highly functional adults, who have been successful in adult relationships and never been in trouble at school or elsewhere, I reckoned we probably made a decent call. Procedure was as follows:
avoid such triggers as you can
use humour to defuse if you can
don't have too many rules
once you have decided you cannot allow something, do not give in or try to purchase good behaviour, accept that meltdowns may have to happen
keep them safe
keep everybody else safe
keep yourself safe
if you can, keep personal and public property safe
don't try to stop the meltdown- it often makes things worse
only cuddle her if that works for her (dd later told me that when she was in a meltdown she couldn't recognise people around her, it just seemed as if a horrible monster was coming to get her)
restrain if you have to (dd used to try to rush at me and bite me which could obviously not be allowed), but do it without anger: imagine that you are restraining a loved one who is in a fever
if a child is trying to bite you it helps to get behind them and hold them by each wrist so you can control where their mouth goes
stay calm
for the time being, your aim is to ensure that nobody gets hurt or frightened
I have memories of endless sessions where I would be holding dd and repeating again and again "no, I cannot let you hurt anybody, no, I will not let you hurt anybody"
That was her security, that made her feel safe: knowing that I could handle the situation at times when she could not.