If you have access to the MySQL database via your hosting provider, query the wp_users table:

SELECT * from wp_users

You can identify those rows by looking at wp_usermeta and comparing the user_id with the ids of rows on wp_users. Delete the row(s) of wp_usermeta that have invalid user ids. You can do this via phpmyadmin.

Note: Backup first.

Additonal note: See the note — backup first!