I have a user who votes 1/5 stars in my site's articles.

I want to find that user and ban him. How can I see who rated articles using 1/5 ? Can you tell me the mysql table and/or mysql command?

Thank you.

You can only see his IP.
If he has a dynamic IP then there is not much you can do.

The IP is stored in the lastip row in the xx_k2_rating table.
You could look for that IP in the xx_k2_users table in case a registered user is using the same IP.