I think it can’t work with CSS but with javascript. I put the content of the zopim-script in a separate file and call the script now only if the screen-size is bigger than 900. Not sure if this is an elegant version, but it seems to work for me. The following code must go in the the head section of your site :
<script type="text/javascript">
if (screen && screen.width > 900)
document.write('<script type="text/javascript" src="https://path-here/js/zopim.js"><\/script>');
</script>