The only way I know of is to use an HTML TextField instead of a Web Forms TextBox and use JavaScript.

The HTML element would look like:
PHP Code:
<INPUT onkeypress="return KeyCheck(this, event);" id="txtTest" type="text"
and the JavaScript:
PHP Code:
function KeyCheck(myFielde)
{
    var 
key;
    
    if (
window.event)
        
key window.event.keyCode;
    else if(
e)
        
key e.which;
    else 
        return 
true;
        
    if (((
key 47) && (key 58)) || (key == 8))
        return 
true;
    else 
        return 
false;