Go to Setup > App Settings > App Designer

This feature allows you to remove menu items that you do not want to appear in the main menu within the app. To remove an item, click on the red X next to the menu item and it will be removed.

Note that some features, e.g. Change Event and Logout are integral to the app and must be visible.

When you are happy with your customisations, click on Save Changes at the bottom of the screen.