dehbuff As Kulga says, that can be (and probably should be) done using .htaccess authentication.
And as he noted in his previous post, this "feature" is already well provided for by the permissions system that's already in place. So I'm not really seeing a purpose to this thread.
farid, please read the FAQ and search existing discussions to learn more about Flarum's capabilities before posting a feature request. If, after doing these things, you still feel that Flarum is missing an important feature that you might need, then take some time to explain exactly what you want and why you think it would be a good idea.
You'll find some helpful hints for effective posting in the Guidelines.