OnBlur Javascript Textbox validation

I have a textbox called txtMobile.text. It offers a list of items for a user to select from a auto suggestion box. This works great until a user goes back to the textbox after selecting the autosuggestion, they can change the text and it doesnt prevent them. Any ideas how i can only get the user to be able to only select items from the auto suggestion and not let them edit the selected text?

<script> 

                //var txtMobile
                var isItemSelected = false;

                //Handler for AutoCompleter OnClientItemSelected event
                function onItemSelected() {
                    isItemSelected = true;
                }

                //Handler for textbox blur event
                function checkItemSelected(txtMobile) {
                    if (!isItemSelected) {


                        alert("Please select item from the list only");

                        document.getElementById("form1").reset();
                        //txtMobile.focus();

                        

                    }
                }


    </script>