Are your authors posting from the normal post editor page or do you have some other system set up for them to post?
If you are using WordPress's standard post edit page, we need to do some troubleshooting to see whether there is a possible plugin or theme issue. First temporarily switch the theme to twentytwelve or Twenty Thirteen and try a post containing these characters. If this is OK, there is a theme issue.
If you get the same result with the theme switched, temporarily deactivate all of your plugins and try and try a post containing these characters. If this is then OK, this would indicate a plugin issue. In that case you can then reactivate the plugins one-by-one checking again each time until the issue reappears to identify the plugin that causes it.