You could use the Azure CLI to stop the the entire cluster:
az aks stop --name myAksCluster --resource-group myResourceGroup
And start it again with
az aks start --name myAksCluster --resource-group myResourceGroup
Before this feature, it was possible to stop the virtual machines via Powershell:
az vm deallocate --ids $(az vm list -g MC_my_resourcegroup_westeurope --query "[].id" -o tsv)
Replace MC_my_resourcegroup_westeurope with the name of your resource group that contains the VM(s).
When you want to start the VM(s) again, run:
az vm start --ids $(az vm list -g MC_my_resourcegroup_westeurope --query "[].id" -o tsv)