Attack of subscribers!

Over the past two days I have 611 new subscribers to my literary magazine WP site. I KNOW they aren’t legit, but how to stop them and what’s it about? I keep getting all the notifications in my e-mail and I go on the site and the number of subscribers has almost tripled in two days.

I assume it’s an attack of some sort, but I don’t know what sort.

Any help much appreciated.