Add this to your <head> section:
<meta name="viewport" content="width=device-width; initial-scale=1.0; maximum-scale=1.0;user-scalable=no" >
There is a more JQM way of doing it which you could add to a mobileinit event handler, but I'll need to dig it out. Reply if you want it.
If I remember rightly, earlier JQM versions prevented zooming by default but the decision was made to enable it around the time the betas started.