Why do you feel you need to prevent it? As a user I'm pretty sure I'd rather just zoom back out if I want to, or use a browser that didn't do this if it really bothered me. But not having the option to zoom is annoying.
I've always been a fan of moving mobile web towards the standards and behaviors of native apps which generally did not require zooming to get tasks done.
Generally I welcome it, because I can click on the teeeny input field and have it come "closer" to me for editing then get out of the way.
Maybe my phone is just a little zealous with the zooming, but it loves to zoom into the top-left corner of the textarea. The first thing I do uses three pinches' worth to zoom back out, and if I ever have to move the cursor and thus touch into the textarea again, it zooms in again.