priviliges for users

Dear,
I have been using webmin to create virtual servers. Each virtual server creation is associated with unix user creation.
THe problem is that those created users are allowed to login via SSH and they are able to "read, via vi-editor, "change directory", via ls  . . . othes's sensitive file. I have check owner of files, their files and folders are correctly set to their own. But, then they still can sneak at each other.
Is there any way to prevent user from executing any kinds of commands out of their home directory?
My default home is /usr/local/apache2/htdocs

Thanks in advace