Submit post: Disable all HTML tags

Hi,

how to make the posted content by the user saved without HTML tags?

I added this in my function.php themes child:
add_filter( 'save_post', 'wp_filter_nohtml_kses');
also:
add_action( 'save_post', 'wp_filter_nohtml_kses');

But it’s not work. 🙁 My user still can submit post by html tags they created and it’s really annoying. They make the font bigger, smaller, then bigest, small again..and so on.. by HTML:
<div style="font-size:50px">blabla</div><div style="font-size:8px">blabla</div> and so on..

Any ways to prevent this?
Thank you.

https://wordpress.org/plugins/wp-user-frontend/