Were you able to find a solution for this?
One of my clients is receiving dozens of spam registrations and there doesn't appear to be a quick way to resolve it.
Unfortunately no, I've just been manually deleting them from my database every so often. The good thing is, you can typically search for an extra bit of code that the spam bots are injecting into the member properties field ("submit":"Register") and that will pull them up for deletion. I also have set maximums on the input name and email fields where the bots are usually forcing a lot of text. You can alternatively search for names and emails with sketchy URLs in them like .ru or other foreign addresses - those are usually spam.