What kind of "users" are you talking about? By default there are a number, "Anyone", "Registered" and "Members" and you may have created more.
You cannot delete users in first two groups, but you can "ban" them which stops the user using that email address. To control who joins those groups you need to have set appropriate permissions, via OPTIONS > USERS > CHANGE PERMISSIONS, before you publicise your forum. You can delete users from the "Members" group because you have put them there.
I don't understand your remark about "email provider" nor about "don't have passwords". The user creates their own password when they register. For more information about controlling member see this post:
http://nabble-support.1.n2.nabble.com/Moderating-free-forum-tp6873076p6873871.html
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.