From your dashboard> Sitemap> If you click the page you want to work with and choose 'properties' this will give you a dialogue box with 3 tabs. click the 'custom attributes' tab. If it's a default page type at the bottom will be an option to 'exclude from nav'. If not - click the drop down back and choose 'exclude from nav' from the list of custom attributes. You could also choose any of the other 'exclude from' options.
If you don't want the page at all then just delete it.