Extract the ninja form plugin’s files to a folder, and use a text editor such as Notepad++ to search within the files of the plugin and find either one of these phrases:
`wp_enqueue_style` OR `css_ninja_form`
You will end up with at least 1 result, which will be similar to :
add_action( 'wp_enqueue_scripts', 'css_ninja_form', xyz );
Which xyz is the priority the author of the plugin used to enqueue it. Now, you can directly remove the line from the result page (not recommended) or you can use the xyz priority to dequeue the script in your function.php.