The whole point of a checkbox is people being able to check it or not...
For sole purpose of passing a form variable with a certain value that wabmaster doesn't want to be seen there are hidden form fields...
Like:
<input type="hidden" name="timezone[]" value="6">
This way you would still get variable $_POST['timezone[]'] with the value 6 like you needed without an user having an abillity to uncheck it...
But again why do you want to use checkbox? Maybe, I didn't get you correctly...