I don't think that will work, the 3 main content sections will not disappear from the page layout page and also I think the master page will break if you remove divRight so css is the only way I think it can be done, and then only on the front end. To remove the layout option would require more than just hiding the right section, it would require removing buttons from the middle section that are used for moving items to and from the right.
Couldn't they just make a business rule and agree not to put content on the right? Seems strange to take away flexibility, sooner or later they will ask for the right back again.
One option might be to move the <asp:contentplaceholder ID="rightContent" runat="server"></asp:contentplaceholder> out of divRight and put it in the footer then just hide divRight with CSS. But then the layout page will still seem like its on the right, though the content placed in the right would land at the bottom. Or if not the footer just put it immediately below <asp:ContentPlaceHolder ID="mainContent" runat="server"></asp:ContentPlaceHolder> in divCenter so it will end up as center content if they do put it in the right.