Spam registrations in WordPress After Migration

So I'm not sure where to put this question... suppose it goes in General then.

I recently moved my website out of multisite, where I had Anti-Slog active. That plugin was apparently amazing, because the moment my site became a single wordpress install at a new server, I began to get hit hard with spam registrations. I've had over 100 new accounts made thus far, and I have no idea how to shut them down.

It's no longer multisite, so I can't use Anti-splog anymore. - So how to I prevent spam registrations? I already have a form of Captcha on the login page - and I'm using Membership Pro to manage my membership site still, and that's fine. However, I cannot stand all these accounts being made like this. I need something to stop them, and I've apparently missed something.

I'm not a fan of heavy plugins that take a lot of resources. A lot of work has been done for excellent optimization of content and such. Is there something that isn't process-intensive that can do this for me, without multisite? Thank you for your time.