Hi @tonyi,
You can try this plugin: https://wordpress.org/plugins/bulk-delete/
Also, please read this topic, especially the #1 and #3 points:
https://wpforo.com/community/faq/how-to-stop-spam/
Are those verified users (they have email verify their account) or just registered?
They did all verify all the emails which is what's wild, with askimet and wordfence installed and the captcha on. I set the forum to require manual approval from now on and was able to delete the users with the bulk delete plugin, although I had to go a few thousand at a time as more crashed the plugin so it took a while. But problem solved for now anyway, thanks so much for the init! 🙂
@tonyi Well if what you say is correct, this can't happen. I mean something is not working correctly or is miss-configured.