How do I block an email in my forms?

I have a simple PHP contact form. But I have email spammer that keeps filling out my form. How do I add a block so that email cannot even be sent from the spammer?
Here my code:

Code:

<?php


// get posted data into local variables
$EmailFrom = Trim(stripslashes($_POST['EmailFrom'])); 
$EmailTo = "smurfmebaby@yahoo.com";
$Subject = "Our Form";
$FirstName = Trim(stripslashes($_POST['FirstName'])); 
$LastName = Trim(stripslashes($_POST['LastName'])); 
$Phone = Trim(stripslashes($_POST['Phone'])); 
$Reason = Trim(stripslashes($_POST['Reason'])); 
$IP=$_SERVER['REMOTE_ADDR'];

// validation
$validationOK=true;
if (Trim($EmailFrom)=="") $validationOK=false;
if (!$validationOK) {
  print "<meta http-equiv=\"refresh\" content=\"0;URL=oops.html\">";
  exit;
}

// prepare email body text
$Body = "";
$Body .= "FirstName: ";
$Body .= $FirstName;
$Body .= "\n";
$Body .= "LastName: ";
$Body .= $LastName;
$Body .= "\n";
$Body .= "Phone: ";
$Body .= $Phone;
$Body .= "\n";
$Body .= "Reason: ";
$Body .= $Reason;
$Body .= "\n";
$Body .= $IP;
$Body .= "\n";

// send email 
$success = mail($EmailTo, $Subject, $Body, "From: <$EmailFrom>");

// redirect to success page 
if ($success){
  print "<meta http-equiv=\"refresh\" content=\"0;URL=thanks.html\">";
}
else{
  print "<meta http-equiv=\"refresh\" content=\"0;URL=oops.html\">";
}
?>

Do I just add something in the Valdiation?

[Free WordPress Themes]

use recpatcha or check this tutorial
http://www.finalwebsites.com/tutorials/php_ajax_contact_form.php

Thanks, that was good info to read. But I am looking for more of a block that won't allow a certain email to be used in the form. Like an @gmail.com. So if they add name@gmail.com. It will give them the error page. Is that possible? I guess I am tryiing to make @gmail.com a banned or bad word?

[Free WordPress Themes]

Thanks, that was good info to read. But I am looking for more of a block that won't allow a certain email to be used in the form. Like an @gmail.com. So if they add name@gmail.com. It will give them the error page. Is that possible? I guess I am tryiing to make @gmail.com a banned or bad word?

what you're trying is not the solution, after you block his address he will use the next one, you need to stop BOTs. You need something else like a captcha validation

Oh I see now. Stop it all and they can't change the email and do it agian. Thanks so much for your help.

What about using e-mail confimration. If you do this it will not matter if he signs up. He will not confirm the e-mail and you have nothing to worry about. I do this and so do my clients. Capthca on an e-mail form is a pain and can sometimes cost you less web savvy readers.

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