This should be what you’re looking for – put this code into your theme’s functions.php file:
add_filter( 'tiny_mce_before_init', 'yourprefix_tiny_mce_before_init' );
function yourprefix_tiny_mce_before_init( $init_array ) {
// filter styles:
$init_array['theme_advanced_styles'] = "your_style=your_class";
// filter formats:
$init_array['theme_advanced_blockformats'] = "p,h3,h4,h5";
return $init_array;
}
This way the only style that will be displayed is your_style.
The 3rd line is taking care of tinymce formats – might be useful too.
See Plugin_API/Filter_Reference/tiny_mce_before_init for more informations and examples.