Help - something keeps adding Superadmin's to my site!

As subject, and it's quite worrying.

For a while I kept getting what looked like spybot member registration, the email address looked obviously "generated". But over the last few days i've seen two new Superadmin logins added to the site. I'm the only user Superadmin for my site, and i'm pretty confident the passwords haven't been leaked. There's been no damage done, but I need to know how it's happening.

I recently infused the TI - Blog System infusion, but otherwise no major changes have been made.

Could someone direct me to a way I can find out how this is happening, and how I can stop it. Or if not, then how I can go about changing all admin passwords to protect the site.

thanks!