What @user4891 said...
I have an autistic DS who has very high support needs at school, including lots of what's called 'challenging behaviours'. This is not because he's a bad child, it's because the primary school environment isn't especially autism-friendly (they're trying).
I don't get those behaviours at home (much) and it's not because I'm an amazing parent, or he gets away with murder, it's because he's under less environmental/sensory stress at home - so it's easier for him - and because I have a variety of strategies to work 1-to-1 through his neurodivergence.
On Saturday, he didn't want to do his homework. We sat for 2 1/2 hours working through strategies, including Plan B from the Explosive Child (which didn't work), during which he panicked and tried to trash the front room. Eventually, remembering that the school educational psychologist mentioned possible PDA (Pathological Demand Avoidance), I read some PDA strategies and tried one of them (using a soft toy to make indirect requests). Immediately, he did all his homework, as good as gold.
At no point was he locked up in a room and deprived of anything. If he's deliberately horrid to his little brother, he sometimes loses a soft toy for two hours, but that's about it. Mostly, his behaviour is great, however, and that's because we MAKE IT EASY FOR HIM TO DO THE RIGHT THING and, unlike a busy primary school, we have the resources to make it happen.
We did all the strict parent stuff, naughty step, blah blah before he was diagnosed because his kindergarten thought we didn't set boundaries, were bad parents, etc. etc. It didn't work and, in fact, it made his behaviour worse.
If a child is neurodivergent, you need to find out HOW they are neurodivergent and try strategies designed for the neurodivergence. Likewise, if they are experiencing emotional trauma, you need to identify the root cause and try to clear it up. Either could be happening here, so the first step is to rule out neurodivergence before considering mental health.
The most important thing to remember is that, if a child has a rock-like strong will (as my DS does), then you have two choices - go around the rock or actually cut through a rock. If the aim is to get to the other side, safely, then it's a heck of a lot easier - and less damaging - to find a way around the rock than to escalate like @ColonelDax until you end up trying to win a battle of wills with a similarly-determined kid.
Because, yes, the parent will win, but it's like nuking the entire world - you win in theory, but you don't gain much and you leave a lot of damage. I don't need to win a battle of wills with DS because, oddly enough, by understanding his neurodivergence, I can GET HIM TO DO WHAT I WANT, with limited stress and without escalating. This is because I am a grown-up who can read online resources about PDA, and he is a six year old🙄