Hi @wang jialu ,
Looks like currently there is no cmdlet to stop cluster using Az Ps as these are the Az Aks cmdlets that are currently supported. If interested, you may raise feature request here to add Az Ps cmdlet to stop cluster. In general, Azure feature team would check feasibility of a feature request, prioritize against existing feature backlog, add in roadmap as appropriate and would announce and/or update the related Azure document once a feature request is addressed.
Until then, as a workaround, you might leverage this REST API for the same and then use this REST API in PowerShell runbook with the help of Invoke-RestMethod cmdlet.