For the record, we get a ton of these too. It just simply happens organically. From what I can tell on our instance, they are all manually entered, but I don’t have a lot of stats to back that up, other than, I haven’t seen hordes of them sign up in a short period of time.
Are the accounts verified/activated? Did they follow the link in the email and then press the button to validate themselves? If they didn’t, they should clear themselves out after 7 days. But if they did, you either can ban them without the posting, or wait for them to post and hope Akismet/Needs Approval picks them up and ban then after their initial post.
That choice is purely yours.