We recommend you not block any browser because that is website suicide, but if you must block a crappy browser for whatever reason:

<?php

$agent = $_SERVER['HTTP_USER_AGENT']; // Access what browser they are using

if (preg_match("/\bMSIE\b/i", $agent)) {

   echo 'You are using IE, not sweat good buddy, you can pass.';

} else if (preg_match("/\bFirefox\b/i", $agent)) {

   echo 'You are using Firefox, you suck, goodbye.';
   exit(); // exit the script after telling them they suck for using a crap tool for browsing the web 
  
}

To find out the AGENT name for any browser just visit this script yourself while using the browser you wish to block.

<?php

$agentToBlock = $_SERVER['HTTP_USER_AGENT']; // this will be the agent you want to block

echo $agentToBlock;

?>