Recently I created one Forum and started facing lot of spam registration and new spam users. I finally controlled by giving moderation through email approval from Admin Panel. Now I am free from spam users.
Well, forum spam registration is the main issue faced by most forum owners. You can try enabling captcha or requiring new users to answer simple questions during registration. However, this would only reduce spam registration and not eliminate them. It require a combination of measures to successfully eliminate spammers, even so it would be difficult.
May I ask what forum software you are using? I know MyBB has a java bot protection plugin that will stop bot registrations and help keep spam to a min. I am sure other forums software have a similar plugin.