css

If you can provide a link to your site and explain what needs changing?

In your theme’s header.php most probably, find <div id="menu"> which probably either has wp_list_pages or wp_page_menu with an additional
<li>for the RSS link. Comment them out or delete them.

Paste the code here from <div id="menu"> to closing </div>of menu from you header.php so I can show you where to delete or just comment out.