spam registrations

I’ve begun to receive spam registrations at my site. What I mean by this is that somehow someone/something is registering on my site. This is impossible to do for most people since I have registration turned off and do not have a registration form on my site. Yet I keep receiving emails informing me of a new user registration.

How do I stop this?

I have looked for plugins, but most only stop spammers from filling out the actual form, using all sorts of tools and secret codes that work for legitimate people trying to register but not so convenient for spammers. But I don’t need that as I have registration turned off.

I have installed Stop Spammer Registrations, Sabre and WangGuard and I am still receiving these new user registration emails.

Is there a way to stop them? A plugin? A setting or hack in WordPress?

I don’t think these idiots can hurt my site, but the emails are annoying.

Thanks!