I installed a forum for a friend and either me nor him managed it ... and now there are 20 000 topics and 5000 users with nonsense content (SPAM).
The forum can I delete with "one click" ... but the users?
Is there an option to show all users at once - or at least 100 or in WP ... this would help, too.
First, you should protect your website and forum, here is a good instruction for you: https://wpforo.com/community/faq/how-to-stop-spam/
Then you can delete users with 100 or 500 count at the time using the Screen Option tap on the top of the Dashboard > Users admin page:
Thank you !!