Sorry mate, I think the developer on this whole plugin has gone.
A rewrite of this plugin is on my list... but its getting to be a long list.
What user role is configured as your default user role?
I'd set this to subscriber, or something very low... so that new users cant automatically write new posts. If subsrciber is not strict enough, download a 'roles' plugin that lets you add a new role that is very strict. make it your default role. It's added effort, but if real users signup, you then just need to set them to the appropriate role.
See if you can add an anti-spam to the form.
These arent always effective, but maybe it will stop some.
If you have IP addresses that constantly hit you with SPAM, then block them via the htaccess file. http://supportcenter.verio.com/KB/questions.php?questionid=504
Hope these suggestions help.