Wordpress multisite getting lots of fake user accounts

Hi There,
I have recently enabled the signup (user and blog) form on my multisite network and as a result I seem to be getting a lot of spam/fake users signing up (just for the user account - not for a blog) - i'm talking about 20 per day.

I think the issue is that the signup form for a MU blog is in 2 parts, the first is to create a user account and the second for the blog - the second part of the form is protected with Google ReCaptcha, which would explain why I'm not getting any spam blogs signed up - but it's frustrating and a little bit concerning that I have all the fake user accounts being created.

I use the anti-splog plugin, Is there a way to add human verification to the first part of the signup form?