I am trying to disable the user from being able to go to a new line in my text area. Why doesn't return false work?

Code JavaScript:
if (is_key(event, 13) && (with_field.value.length > 1)){					
	var search_list_ele_array = new Array();
	var search_list_ele_array = document.getElementById('with_auto').getElementsByTagName('li');
 
	for (j=0; j<search_list_ele_array.length; j++) {
 
		if ((search_list_ele_array[j].className == 'user list_hover') || (search_list_ele_array[j].className == 'comp list_hover') || (search_list_ele_array[j].className == 'blog list_hover')){
			var link = search_list_ele_array[j].getElementsByTagName('a');
			user_add(link[0], search);
		}
	}
 
	return false;
}