It's an IOS feature, and it's not preventable unfortunately. There are two ways to get around it globally.
1. Setting the input font size over 16 pixels. The mobile safari then will decide not to zoom in, as it's sufficiently big. (plugin Theme Options -> Input Field Layout -> Search input font option)
2. Using a no-scale meta tag in your site header. You can try putting this meta tag to your site header.php file:
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" />
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