You know what I really think you should do with a kid like that? Put him on your team! Yes, what he is doing is dangerous but it shows something else as well. It shows he is one of those fearless kids who is willing to try anything, it shows he is a kid who enjoys challenging his body and testing his limits, it shows he has a good aerial awareness and a sense of himself physically. Just the kind of kid that is born to do gymnastics!
I am not saying you should except dangerous behaviour and not following instructions, but tones that just seem to not be able to help themselves to try things like this often end up being your most talented gymnasts.
Now a few tips for the behaviour.
1. You need to make expectations clear, you know that this behaviour is dangerous but this child does not know it, he doesn't have the forethought to think "Gee, I might land on my head and break my neck". He only knows its fun, punishing for doing this without letting him know it's not okay won't help because he won't know what he did wrong. If you see this behaviour the first thing to do is stop the whole class and sit them down and explain why. In most cases you won't see it again.
2. 21st century kids are different, they are not taught to blindly follow the instructions of an adult. In many ways this is a good thing, back in the old days kids were often a bused because they were taught they had to do what an adult says no matter what. Kids are taught to think for themselves, so we need to discipline them with this in mind. It's very important for kids to know why something is dangerous.
So once you establish to the kids what the expectations are and why they need to follow them, sure, you will still have kids who defy you, the next steps are as follows.
1. Praise the kids doing the right thing. A lot of time kids do the wrong thing because they forget and get distracted or because they want attention. One great way to combat this is to praise the kids who do the right thing. Look for kids doing the activity correctly and say things like "Wow, Johnny is so strong he can make it the whole way across the bars without bending his arms". Or set it up as a challenge "it's really hard to get the whole way across without bending your arms, I'll be so impressed if anyone can do it". The kids who want to challenge themselves physically or who want attention will quickly do the right thing.
2. Redirect the behaviour. Instead of focusing on the wrong thing then try to encourage them to do the right thing. Like "Billy come and show me how good your front support is". Or "Dam I want to see how far you can get across this bar in front support".
If these methods fail obviously you need to discipline the child, but the way you did it could land you in a lot of trouble. I notice that you are from Australia and there are certain laws and expectations when it comes to physically handling children. Reasonable force is allowed only to the point to stop the child hurting themselves or others. Dragging them off the equipment and plonking them at your feet and dragging them back to your feet when they move could be construed as innapropriate. You don't want to be sued.
Some suggestions for discipline.
1. Once you establish that the child is being deliberately defiant and can't be redirected with positivity. Change your tone of voice and redirect. Ie "James, that is not the correct activity, you need to go to front support and walk across on your hands". Use a very firm and low tone of voice, the change in tone will let the child know you mean what you say. Its best to tell them what you want them to do, so that is the picture in their head rather than tell them to stop what you don't want them to do.
2. If the behaviour continues issue a firm warning "Jack, I have told you to do the activity correctly, if you continue to behave in a dangerous way you will have to go and sit out". Firmly state exactly what the consequence will be.
3. If it continues issue a time out. Your feet is probably the worst possible place in the gym for time out. The child can continue to get you attention by squirming round and crawling away. They can continue to get attention from other children, they can get overstimulated by the equipment and the other kids. Time out should be a spot away fro. The group, away fro. The equipment where they can just sit, but of course somewhere you can still see them.
4. To get the child to go to time out, don't drag them, use firm eye contact and a firm voice. If they refuse one of the best things to do is start them focusing on what they can do to get out of this situation. While they are refusing they see this as the only way to get out of it. What you can do say is "Jack, I've told you to go to time out, show me that you can listen and do the right thing and I will ask you to join back in". This way the child can focus on doing the right thing and improving their behaviour.
Sitting them out for the rest of the lesson is unhelpful. Because they will get bored very quickly and start to disrupt for attention, making it very hard for you to coach. Also it does not give them a chance to learn fro. Their mistakes and fix their behaviour. If they feel like they have messed it up and there is no coming back to class, they will have no quarks about continuing to misbehave as they have no good reason not too.