you can write this in yourself

Example:
<?php
// this would be a simple ns4 test, say to protect
//some page code that will not work in Netscape 4x
if ( browser_detection('browser') == 'ns4' ) { execute some code; }
else { execute some other code; }
?>

2 ways of doing this. go to .htaccess and then change index.php for default directory to a php file named whatever you want. add this code and modifty it to your liking...i recommend sending them to a page that says sorry we do not support your browser.