I used a customized response required that virtually stopped spam registrations. Most of these people are foreigners from China and eastern block countries... they don't speak English, but can type in random letters. That won't stop them... tried that.
Ours requires an answer to a question like "what is your favorite hobby?" Duhh... sailing. Or where can you use a sailboat... water. What is the name of the website you are visiting?... Catsailor. You can add multiple answers that fit.
They can't seem answer these questions.
Next... moderate the first 1-2 posts of new users.