Best thing is change your WORDPRES LOGINS man. Try these steps Open your cpanel > PHPmyaadmin > wp_user option > and change the PASSWORD keeping MD5 option. Now no one can login to your wordpress dashboard and if you have any GUEST writer simply DELETE their account that's it.
somebody hacked you admin account for the site and is posting with it , change password , check pc for any keyloggers , change email password and scan through your hosting server for any unwanted files
Author's are allowed to publish articles in WP. Change their role to either subscribers or contributors, if you don't want to give them write to publish