Removing from the sitemap
You can remove a page from the sitemap (/sitemap.aspx) which is usually registered with Google Search Console.
Click the Advanced Tab, then the SEO option and change the Sitemap Priority to < 0.5.
NOTE: Google no longer uses the Priority setting in the sitemap when it is indexing the page.
If you don't want Google to index a page you can also switch off the "Allow indexing" option.