If you want to remove specific element of your prototype without breaking anything, you can use the navigator to access specific element inside your mockup. For each element in the navigator, you will be able to remove or copy that element.
To access the navigator, click on the Navigator icon located on the Action Toolbar.
Using the navigator
You can navigate through your design elements. To delete an element, click on the trash icon. To duplicate an element, click on the copy icon.