Sometimes Lockdown of your browsing device is incomplete without disabling zooming option. Zooming in and out with multi-touch gestures such as pinch-to-zoom can be disabled using kiosk browser like SureFox however the user would still be able to zoom using 'double-tap to zoom' feature.
And if you want your webpage to have disabled 'double-tap to zoom' feature, you can use following Meta Tag.
Disabled Double-Tap to Zoom