Who has access to your log-in information and/or email address?
Did you use the same password for your email account and your blog?
Are you sharing log-in information with anyone or leaving it where anyone can locate it?
Are you remaining logged in on your computer so anyone can come along and access your blog through it?
Go to your email program and change the password to a very difficult one because that's how many hackers gain access to blogs
Then read this please knowing that blogs don't get hacked when security protocols are followed. > 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
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.
I tagged this thread for Staff. Please subscribe to it so you are notified when they respond. Note most support Staff are at a meet-up and are monitoring threads so they may not get to this one until after the weekend.