List all users whose "member for" and "last access" are the same
These won't be exact matches.  It uses a timestamp value that will vary, even if only a few seconds.

I would suggest doing this in some custom PHP

that module, or writing your own custom functions.  you'd convert the timestamp difference to a reasonable value (say, within 1 hour?) and then user_delete()

I would also add a captcha and/or email verification to help prevent this.