I know there are threads about how to get rid of splogs. But let's face it - nothing really works.
I run the reCaptcha plugin and it doesn't seem to stop the spam signups at all. I start every day at work by going into my WPMU sites to remove new splogs.
Maybe if everyone suggested functions for an anti-splug plugin, the kind people here at wpmudev could look at making a new premium plugin. Seems like you've had problems yourself with splogs and I think it's something that everyone running wpmu has problems with.
The .htaccess approach didn't work at all to me. I don't know if I did something wrong or not but it just didn't work.
I've noticed that almost all splog signups sign up with a name ending with numbers. i.e. name1984. Maybe blocking people from using a username ending with 4 numbers would be a good idea? I mean it must be rare for normal users to select that kind of name.
Maybe the splogs are often signed up from a specific client, and the use of name1234 is blocked for that specific client?