How I can delete a forum completely with user and topics/posts.
(I am using multisite, so want to remove site 2 forum completely with users and all data)
I tried to "unistall", it warns about removing of data but in network users are still there.
Users don't belong to wpForo, users belong to WordPress, and they should be deleted by WordPress. All users are stored in WordPress tables, and they are under WordPress core functions control.
wpForo deletes all its database tables and content, but it doesn't touch users. You should ask "How to delete certain sub-site users?" question in WordPress support forum. You may find some plugin or some script: https://wordpress.org/support/forum/multisite/