How to stop splog signups

The plugin installed and I am enquiring about is Anti-Splog.

Why is this installed plugin not in my list?

Our site is in development and I'd like you to help educate us as to how we are getting so many splog signups per day after such a short time.

We clearly state that signups are monitored and splog user and accounts are promptly deleted.

How are they finding the MultiSite community?

Are they using automated software to signup and if so what is it likely to be - so we can find a way to stop it?

It would appear that once our site is live and (hopefully) popular it could be a full time job deleting crap accounts.