Add the following meta tag in your HTML header of your index.html:
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no"/>
user-scalable=no should be sufficient, but either iOS or Android needs initial-scale and maximum-scale, I think.
As far as I know, you can’t manipulate this (or any other) browser setting in your Mendix app. This is a security precaution put in place by all browsers I am aware of.