I'm using Redhat OS.

how to block the user access or open his website via http://ip_or_our_host/~user

One way is this

There will be a directive called UserDir in httpd.conf which gets appended to the user's root represented by ~

For example if you are storing thisuser's web pages in /home/thisuser/www then you can put UserDir www for enabling the access http://userssite/~thisuser

Just put a different directory there instead of www and it will return a 404

Cheers
Balaji

Disable mod_userdir on your httpd.conf.