Hiding SharePoint Ribbon Control from users

Add this to the head section on master page.

<style type="text/css">
div#s4-ribbonrow {
display:none;
}
</style>

<Sharepoint:SPSecurityTrimmedControl runat="server"
Permissions="AddAndCustomizePages">
<style type="text/css">
div#s4-ribbonrow {
display:block;
}
</style>
</Sharepoint:SPSecurityTrimmedControl>

This should hide the ribbon control.