How to ignore the specific CSS codes coming from the WordPress plugin stylesheet?

I'd say you should check the handle of the plugin's stylesheet. Look into the wp_enqueue_style part and find out that handle, and then add your own styleshet AFTER that handle by using that handle as a dependency. For example, put this in your theme's function.php:

wp_enqueue_style( 'your_own_handle', 'path/to/your/overwrite_stylesheet.css', array('handle_of_plugin1','handle_of_plugin2'))

With that done, you could overwrite all the plugin css either in whole or in parts.