Stop 'enter' key submitting form when using jquery ui autocomplete widget

You can use an event handler on it.

$("#searchTextBox").keypress(function(e) {
    var code = (e.keyCode ? e.keyCode : e.which);
    if(code == 13) { //Enter keycode
        return false;
    }
});

see: jQuery Event Keypress: Which key was pressed?

also: http://www.cambiaresearch.com/c4/702b8cd1-e5b0-42e6-83ac-25f0306e3e25/javascript-char-codes-key-codes.aspx for list of keycodes

$("#autocomplete_field_id").keypress(function(event){
  var keycode = (event.keyCode ? event.keyCode : event.which);
  if (keycode == '13') {
    event.preventDefault();
    event.stopPropagation();    
  }
});