i also tried to remove the user from register group and it works...
Do NOT remove users from the "Registered Users" group. You will cause more problems down the track.
As Brf said, to stop a new user from accessing a forum, give them a NEVER permission on that forum.
If you are confused about what the total permissions applying to one user are going to be, use the "Permission masks" feature on the "Permissions" tab in the ACP on that user. It lets you see what the final permission is, and how it got that way.