Go to your email program and change the password to a very difficult one because that's how many hackers gain access to blogs
Read > http://en.support.wordpress.com/security/
1. If you can log-in go here > Users > All Users and delete any user that does not belong there.
2. Disable post by email > http://en.support.wordpress.com/post-by-email/
3. Disable post by voice > http://en.support.wordpress.com/post-by-voice/
4. Change your blog password to a very difficult one > http://en.support.wordpress.com/passwords/#change-your-password
You can also reset your password via your Settings tab on the WordPress.com home page:
5. Use a secure, encrypted connection to connect to your Dashboard. Under Users → Personal Settings, check the box that says “Always use HTTPS when visiting administration pages, and click Save Changes.
P.S. I flagged this thread for Staff help. Please confirm the URL of the blog you are referring to if you have not already done so.