I have try with Question & Answer Verification and reCAPTCHA and it kill spam but when I remove it and put just simple picture new registrations jump 150% (2% was spammers ) so I was thinking what to do and as Trip already said moderate new members is the best way.
Make new user group where will be your real members with 1+ posts and put moderate posts for current default user group. After that go and make promotions that after 1 post user from default user group is promoted to group where is you real members.
For every new user you will need to moderate only first post, after you aprove it he will be in regular user group and will continue to post normal.
Hope it help