As you might know, it was not possible to stop your AKS clusters completely because system pools are always required to be running. I have posted about this like below in past.
normalian.hatenablog.com
Here is quite useful feature to reduce AKS cost much effectively than before.
docs.microsoft.com
It should be really easy to follow the article. I have open Azure Portal and try to stop my AKS cluster like below, and it seems to be fine even if you have never registered "extension aks-preview" like below.
You can find your node pools will be 0 nodes on Azure Portal or command lines after you have completed the commands.