Good luck trying to stop them. Gymnasts need to be upside down and as a coach you know they will show their skills to their friends on the school yard and you know they will try things on their backyard trampoline. The best you can do is talk to them about safety, if they are working on new skills let them know they aren't ready to try them on their own yet.
One of the biggest things we have to be careful of as a coach is not being over protective, this can cause more problems. If we are constantly holding kids back and not being prepared to teach them the skills they are ready to learn they will teach them to themselves, and this is when it gets really dangerous. We are better off teaching them a little sooner but in a safe way in the gym.
Telling kids not to do things at home doesn't work, teaching them about safety does. Teach them to be aware of their surroundings when they do skills, teach them to read the signs their body is giving them of being too tired or not concentrating enough, teach them to understand why we don;'t allow them to go home and try their brand new skills straight away, teach them to learn to understand their skills and feel when they are really not ready to do them at home.
We have all had many kids who come in the gym and throw all sorts of flips often on the trampoline. The coach who just growls at them and says "don't do that its dangerous" is the one most likely to have an injured kid. The kid obviously does them at home and will continue to do so if they don't get that outlet in the gym. The coach who turns around and says "OK, lets work on that and see if we can make it a bit safer" will have the safer gymnasts in the end.