Most spam sign ups are done by bots that are able to overcome your typical captcha.
On DN.ca we simply added one word to the sign up proocess and it stopped the bots cold. They are programmed for your typical Xenforo installation and as soon as you change it the bots are stopped and it takes human intervention.
At DN.ca we simply added a requirement to type in a word and if it ever gets programmed into the bots we simply change the word.
This feature is a part of Xenforo and takes only moments to activate.
Right now I see no way for a new member to join dntrade and that is unfortunate, it is such a good resource.
PS. We link to dntrade on the main page of DN.ca, so do some of the other Domain Forum Friends, you may want to consider doing the same.