The setting gear icon/button is located on the app homepage on the top right-hand corner. The gear icon color can be customized on the Design page under the Navigation tab so you can hide it from your app users.
You needed to hide the settings gear so that your app users will not be able to see it and thus cannot use it to change their account settings (eg password).
Note: There is no feature for disabling/deactivating it. As a workaround, you can hide it by matching the color of the icon with the background color.
1. Logon to your Dashboard and select the app you wish to change the UI bar back button for
2. Right-click on it and login
3. Navigate to Create -> Design -> Navigation Tab and locate the Homescreen Shortcut Color setting
4. Set the color on the palette the color that matches your background color
5. Click Save on the top right-hand corner
You can see the change using the CMS embedded previewer on the right-hand side of the page.
For example, if you have chosen 'grey color', the settings icon gear will not be visible: