locate the file index.php in your template folder then open the fie using any text editing program. Then go to line 75 and you should see this code
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=3.0, user-scalable=yes"/>
The <meta name="viewport" constrains your page down for mobile devices. You can remove this part. And if you also want to remove the responsive functionality then go to line 66 and look for any css or .js or containing "responsive", "respond," or "resp" and remove that.
make sure to make a backup of the index.php before you make any changes.