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.