I want to block or delete all my mails of my gmail account as someone has hacked my password and I am unable to access my account.Plus that person has now changed my id also and he is going to misuse my account.So for security reasons I want that my account gets blocked,so that no one is able to access it.Please help me out??

Contact Gmail - no one else here can help you:
Gmail: Help Center (http://mail.google.com/support/bin/request.py?contact_type=contact_policy)