bogus subscribers

I keep getting new user notifications for bogus subscribers.  I have been deleting them.  Here's my most recent example below.  Many come from the .ru domain. I added a plugin WPBruiser which helped for a while, but in the last 2 days I've had about 10 new fake subscribers.  Is there anything I can do to stop bogus users?  I am assuming this is a stupid hacking technique.  Why do people fake subscribe?