First of all there is no isolation for single operators in Mibew, but there is groups isolation.
Next, you can use GET parameter named "operator_code" to initialize chat with a specified operator. This parameter should be appended to URLs in button code. For example, if you have the following button's code (text link button):
<!-- mibew text link -->
    <a id="mibew-agent-button" href="http://<Your Domain>/mibew/chat?locale=en" target="_blank" onclick="Mibew.Objects.ChatPopups['55719c791fe9c955'].open();return false;">
        Start chat
     </a>
     <script type="text/javascript" src="http://<Your Domain>/mibew/js/compiled/chat_popup.js"></script>
     <script type="text/javascript">
         Mibew.ChatPopup.init({
             "id":"55719c791fe9c955",
             "url":"http:\/\/<Your Domain>\/mibew\/chat?locale=en",
             "preferIFrame":true,
             "modSecurity":false,
             "width":640,
             "height":480,
             "resizable":true,
             "styleLoader":"http:\/\/<Your Domain>\/mibew\/chat\/style\/popup"
         });
    </script>
<!-- / mibew text link -->
you should replace it with the following
<!-- mibew text link -->
    <a id="mibew-agent-button" href="http://<Your Domain>/mibew/chat?locale=en&operator_code=<YOUR OPERATOR'S CODE GOES HERE>" target="_blank" onclick="Mibew.Objects.ChatPopups['55719c791fe9c955'].open();return false;">
        Start chat
     </a>
     <script type="text/javascript" src="http://<Your Domain>/mibew/js/compiled/chat_popup.js"></script>
     <script type="text/javascript">
         Mibew.ChatPopup.init({
             "id":"55719c791fe9c955",
             "url":"http:\/\/<Your Domain>\/mibew\/chat?locale=en&operator_code=<YOUR OPERATOR'S CODE GOES HERE>",
             "preferIFrame":true,
             "modSecurity":false,
             "width":640,
             "height":480,
             "resizable":true,
             "styleLoader":"http:\/\/<Your Domain>\/mibew\/chat\/style\/popup"
         });
    </script>
<!-- / mibew text link -->
The operator code can be set at operator's profile page.