You need to use JavaScript to handle this. I would typically look at using jQuery to make this functionality a lot easier, then you would hook up like this:
$(document).ready(function() {
$('#allTextBoxesToWatch').keypress(function(ev) {
if (ev.which == 13)
ev.preventDefault();
});
});
In this example (which I've just typed from memory, so I apologise if it isn't 100% exact), it binds all textboxes that have a class of allTextBoxesToWatch to the keypress event. It then determines whether or not the user pressed the key with an ascii value of 13, and prevents the default action if it does.