This may not be enough, but it can be solved by changing below settings of the admin panel.
Admin > Spam
- All new users must confirm their email: ON
- Enable moderation (approval) of users: ON (This will be difficult to operate.)
- Use captcha for user registration: ON
- Use captcha if email not confirmed: ON
- Use moderation if email not confirmed: ON
- Use moderation for users with few points: ON
Admin > Permissions
- Asking questions: Registered users and email confirmed OR Registered & email confirmed & enough points
- Answering questions: Registered users and email confirmed OR Registered & email confirmed & enough points
- Adding comments: Registered Registered users and email confirmed OR Registered & email confirmed & enough points
If these basic features do not eliminate much spam, you may need to install any anti-spam plugin. You can find some plugins by searching this site. However, those plugins are not all-purpose. You should be satisfied with some reduction in the number of spam.