New User Registration and Spam Postings

Hi, I repeatedly have new users register with one of my wordpress sites (practiceimprovement.com.au) and make a number of spam postings (about video convertors).
This is despite the fact that new user registration is disabled.
When I delete the spam user and their posts, within 24 hours they are back again!
The username for the site admin is not ‘admin’, I have updated passwords and changed the mysql username and password.
Any suggestions?
Thanks,
Tony