Bruce Chapman wrote:
@matthew - the registrations are obviously using some type of list of harvested email addresses, which is why they look like real users to the uninitiated. They also use generated usernames which look kind-of-real.

This is the script I used to soft-delete all the users that were spam registered:

update Users set IsDeleted = 1
where UserName = displayname and (LastIPAddress is null or LastIPAddress = '') and FirstName = '' and LastModifiedByUserId = -1
and CreatedOnDate > '2014-05-12'

The date was one in which I detected the spam registrations starting - this might be different for different sites. The definining signature of the spam registrations is the lack of activity after the registration. I would use a select statement with the same 'where' clause to see if it is picking up only the bot registrations and leaving your actual users alone.