In functions.php file add this

add_filter( 'body_class', function( $classes ){
foreach( (array) wp_get_current_user()->roles as $role ){
$classes[] = "user-role-$role";
}
return $classes;
});

and in your css file

.user-role-customer div#mceu_20{
display:none;
}

edit: all users in my forum are customers (woocommerce)

it would be great to be able to publish an iframe without using codes, I hope for this update.

yes, these changes will be included in the next version release. 

I have only one more doubt. I do not like to allow all users to have permission to publish html codes, would not I have some way to make this possible only for admin and moderator?

I'm sorry, but wpForo doesn't have such an option. However, wpForo has a built-in Spam Control which has a "New Registered User" control subFunction. The "New registered user" status is the same "moderation" status.

You can manage the options the Dashboard > Forums > Tools admin page.

it would be great to be able to publish an iframe without using codes, I hope for this update.

I have only one more doubt. I do not like to allow all users to have permission to publish html codes, would not I have some way to make this possible only for admin and moderator?

Currently I disabled with css but this is applied to everyone.

First of all please add the src, width, height attributes with the iframe tag:

iframe(src width height),

Probably you're trying to post the only video without any content, right? If yes, please try to add some text with it. 

We're preparing a new wpForo version this will be fixed. I mean that you'll be able to insert only the iframe without any additional content. 

http://prntscr.com/nk6asy

The preview shows the content correctly, but when I click Publish I get the message "empy post" and nothing is published
http://prntscr.com/nk6ae2