Hello,
Thank you for your query and we are so sorry about the trouble this must have caused.
You need to disable the email reporting feature in your staging website. To do so, please head over to your staging website’s admin dashboard > Rank Math > General Settings > Analytics and under the “Email Reports”, disable it:

As for ensuring your staging website is not indexed, please head over to your admin dashboard again > Settings > Reading and check the option “Discourage search engines from indexing this site”:

Hope that helps and please do not hesitate to let us know if you need my assistance with anything else.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.