Reggie,

The same happened to my forums. Believe me, they are spambots, not humans. I defeated them publishing the required answer in a previous screen in the registration process. Apparently spambots (still) can't solve this kind of challenges.

There was a thing on the news recently. There are offices in very poor countries where people are employed to sign up to forums, use message boards etc to post spam because it is cheaper to employ them than write very clever software to do it.

Which is pretty sad whichever way you look at it.

If that is the true, then my problem was solved because these employees do not understand Spanish. Lucky me.

Blocking the IP is unfortunately of little value these days. The bots change IP all the time, possibly by going through those botnets. They may even try to post via e.g. the 'tor' network, which, if those IPs are getting blocked, would block out legitimate users that may for some reason or another sometimes have to go through an external proxy.