Hide Ribbon control in SharePoint 2010 using css

 The Security Trimmer control can be place around the ribbon in Master page.The PermissionString attribute value can be changed based on the requirement.And replace the code in master page and paste the below code in master page.

<SharePoint:SPSecurityTrimmedControl PermissionsString="ManagePermissions" runat="server">

   <div id="s4-ribbonrow" class="s4-pr s4-ribbonrowhidetitle">

   </div>

</SharePoint:SPSecurityTrimmedControl>