You can use px in IE and the user will not be able to adjust using the browser settings. However all other browsers will let you zoom or whatever so you can't stop it unless you want to use images.
You should try to design so that it doesn't matter what size text the user selects. Use ems or percentages to allow for a fluid page. Of course this isn'y always as easy as it sounds when you need to match things exactly. But with a little forethought it can be done and will help the visitor