Try a task-type of sign-up question instead of "who is this site for?" multiple choice
The question can tell them to go to the vocalists page and type us the name of the person listed between margaret dorn and megumi hayashibara (or something easier or harder)
Forcing email verification has already prevented activation of many spambots accounts (it's visible in the admin control panel), but they are getting past the "who is this site for" question. We'll see if the harder question works to deter some more. This way new members can sign up and post without having to wait.
If that doesn't work, another method would be to hold only new members posts that contain links.
(BTW there is an easier method than deleting every spampost: when you delete a user you can select from the dropdown menu to delete all their posts at the same time)