Annoying auto-commentators

I published a post and within 10 minutes, I had 6 inane auto-comments from people I've never heard of. I've never really had a problem with that before (I've had to delete the occasional comment, but nothing outrageous). I'm requiring sign-ins on the blog and we'll see how that does. I don't want to have to moderate comments, but I can't be spending too much time deleting links to porn sites. I'd rather be doing something fun.