What is best way to stop bot signups? Is Anti-splog for this or only for new blogs?

I am using BuddyPress with WP multi-site. I have about 10 blogs and I do not allow subscribers to create their own blogs.

Recently, I am getting a lot of signups with strange email addresses, typically a name followed by 9 numbers, e.g.,

zaire749080799@bestcoffeemakersratings.com
mathew118993194@americanidolnews.info
etc.

So far, I cannot tell these emails are doing any damage.

1. Should I be using anti-splog to stop bot email addresses?
(I thought anti-splog was for stopping users from making spam blogs.)

2. Is there another recommended plug in for this purpose?

3. Did these signups really respond to the WP BP verification emails?

4. How do I clean out bad email addresses after they are already in my database?

Thanks,

Ed