You can remove a Custom Page by simply clicking on the page you wish to remove and hovering over the dropdown action menu located next to the heading and then selecting delete from the drop down box and confirming when prompted.
Alternatively, you can go to any custom page, and click on Manage Custom Pages in the dropdown action menu, and then in the menu of the custom page you wish to remove, click Delete.
To remove a custom page from the navigation menu, go to Admin>Templates, and click on the item. Then select Delete. Note that this doesn't delete the actual page. You must follow the steps above separately to do so.
* Note that the name given to modules, features and information items may vary in your platform as they can be customised by Main Admin users.