PHP return false if certain word appears in $_POST[xxx]
<p>I have a coming soon form at a website where user fills out an email form and it will be emailed to me. However, a spammer has hit the site and is spamming the form with goatse and so on. IP ban isn't helping so I need to stop the form sending it if it contains goatse or something. Here's the mailer.</p>

<pre><code>&lt;?php
$SPOSTI =$_POST[sposti];


if ($SPOSTI=="")
{
    return false;
}

if ($SPOSTI=="goatse.fr")
{
    return false;
}
if ($SPOSTI=="http://www.goatse.info/hello.jpg")
{
    return false;
}
else
{
    $to = "xxx@gmail.com";
    $subject = "xxx";
    $message = "$_POST[sposti] haluaa tiedon kun kotisivut.name avautuu.
    $_POST[ip]";
    $from = "$_POST[sposti]";
    $headers = "From:" . $from;
    mail($to,$subject,$message,$headers);
}
    ?&gt;
</code></pre>

<p>Is there someway to block it from executing the code if the email contains a certain word (goatse in this case)</p>
