The only directly sitemap-related setting (update frequency) can be found in Community Admin -> System -> SEO. You cannot directly edit the auto-generated content or influence what get's included. The sitemap generation will use anonymous user's permissions though, so if you want to hide areas then make sure to tighten up viewing permissions for that node. See also How to upload a Sitemap in Community
How you can influence page indexing is by editing the robots.txt though. For details on how to edit that read What is the default robots.txt file and how can I access or change it?
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