Dont allow Alphabets and Special characters in TextBox using JavaScript

HI

I use below code for my text box

 <script type = "text/javascript">
         function isNumeric(keyCode) {
             var r = ((keyCode >= 48 && keyCode <= 57) || keyCode == 8 ||
                (keyCode >= 96 && keyCode <= 105))
             if (!r) {
                 alert("please Enter Number");
             }
             return r;
         }

here in Textbox we should type number if we type alphabet it show error but it didn't clean alphabet from textbox.

I mean if users type alphabet in Textbox it just show error when user click Textbox if they didn't clean their alphabet from textbox and click on button that alphabet insert to database

I want when users type alphabet in textbox when it show error it clean textbox .

Best regards