I’ve been searching for an answer, that I couldn’t find here. It was actually quite easy to find the solution by myself. But just in case someone need do to this too, this is how you do it:

-> Create a new folder named “bbpress” at the root of your wordpress theme:
/wp-content/themes/YOURTHEMENAME/bbpress/

-> Find the file named “content-single-forum.php” located here:
/wp-content/plugins/bbpress/templates/default/bbpress/content-single-forum.php
and copy into your newly created folder:
/wp-content/themes/YOURTHEMENAME/bbpress/content-single-forum.php

Now bbpress will use this file instead, if your “YOURTHEMENAME” is in use.

-> Open the file under
/wp-content/themes/YOURTHEMENAME/bbpress/content-single-forum.php
Go to line 48, and you should find this:
<?php bbp_get_template_part( 'form', 'topic' ); ?>
Comment this line this way:
<?php //bbp_get_template_part( 'form', 'topic' ); ?>

And you’re done! Easy and clean!

Of course you might want to add a button to create a new topic.
You can do it, following these steps:

https://bbpress.org/forums/topic/how-to-add-new-topic-button/

[@bobibrown0 Participant 7 years, 5 months ago]

* How to: REMOVE new topic form at the bottom of single form

[@bobibrown0 Participant 7 years, 5 months ago]

Also comment the line 42, which is for when you have topics in the forum

<?php //bbp_get_template_part( 'form', 'topic' ); ?>