Hi,
May be your hyper link is containing the username as query
string. like..
www.mysite.com?user="A" check this.
You can make the users to compulsorily login to your site by introducing your own login machanism and making your log in page as the entry point.
You can also check the session in between and validate the user.
it would be better if you send some code snippets so that it will show clear problem.