I still say some kind of additional field during registration would prevent them from registering in the future. Something where there is only one specific answer. Just have a required question like "Which company created Mario & Zelda". Then the person registering has to put "Nintendo", which an automated sign-up cannot answer because it has an intelligent, specific answer. For even more security, have the question in a picture, rather than as text, so you have a .jpg with the question on it, so automated sign-ups cant even read the question.
Worked wonders on my old sites.
