Yeah, it's using reCaptcha atm. And there wasn't anything overtly spammy about those posts or users, so I dunno what's up. My guess is a delayed thing, where after a few months they then go in an change the signature or text or something like that.
Also, when you ban/delete spam, send me a message about the user so I can keep tabs on spam traffic. When it spikes it tells me I need to rethink spam controls.