Adults are not allowed to hit one another. Of course not. Adults don't do those sorts of things. Older children want to be more like adults. Help them understand what the standards are for grown-ups.
Children find great comfort in predictability.
Establish clear consequences for breaking the rules and always enforce them. However, don't expect children to automatically accept the rule or the consequence for breaking the rule. That is not the nature of school-age children. They may argue about the rule or stomp off in a huff. If you calmly enforce the rule in spite of their defiance, you are helping them learn what is expected of them.