Go To Power Pages Studio
Open Your Site
Click Pages From Left Menu
Select The Page You Want To Hide
Click Three Dots Next To The Page Name
Click Hide From Navigation
Or
Click The Page To Open Its Settings
Uncheck Show In Navigation Menu
Page Will Still Be Accessible By Direct Link
To Restrict Access, Go To Page Permissions
Remove Web Roles Or Set Proper Access Controls
No Code Needed For Hiding From Navigation
If You Are Using Site Map Manually, Use Below XML
<siteMapNode title="Page Title" url="/page-url" visible="false" />
Set Visible To False To Hide From Menu
Publish Site After Changes