Keep main page in the navigation menu and then for your sub pages, you don't put them in the nav menu - open the Inspector and then click on Page and then click on the box that has "don't include page in nav menu" and that is it.
You then create your own links on the sub pages back to the main page and wherever you want by using either shapes or text boxes or whatever.
To make the whole process easier, you could always ditch the iWeb page template nav menu by turning it off and creating your own text based nav menu, which makes creating sub pages easier - you just don't include what you don't want.
My previous post does tell you how to do it. Open the Inspector and click on Page, which is the first one and then in the Page settings there is Page and Layout. Select Page and then uncheck the box that has "Display nav menu" and the nav menu will disappear and you can then build your own text based nav menu.