WordPress disable ‘Install Themes’ tab

You can essentially ‘block’ the capability install_themes using;

function __block_caps( $caps, $cap )
{
    if ( $cap === 'install_themes' )
        $caps[] = 'do_not_allow';
    return $caps;
}
add_filter( 'map_meta_cap', '__block_caps', 10, 2 );