Disable single Wordpress plugin update notifications

functions.php

<?php
// Disable update notification for individual plugins
function filter_plugin_updates( $value ) {    
  unset( $value->response['block-spam-by-math-reloaded/block-spam-by-math-reloaded.php'] );    
  return $value;
}
add_filter( 'site_transient_update_plugins', 'filter_plugin_updates' );
?>

You could also just add this to the plugin's main php file.

<?php
add_filter('site_transient_update_plugins', 'dd_remove_update_nag');
function dd_remove_update_nag($value) {
  unset($value->response[ plugin_basename(__FILE__) ]);
  return $value;
}
?>