Have you already coded the site?
Seeing as there is no limit to how much you allow them to click, in order to prevent them using something like iMacros, you could implement a captcha or some other form of verification.
Not for every click, but say one in 10. A simple captcha though, because people generally fucking hate them.
You said you were using PHP, so heres a captcha you can modify:
//captcha.php
<?php
session_start();
$text = rand(10000,99999);
$_SESSION["vercode"] = $text;
$height = 25;
$width = 65;
$image_p = imagecreate($width, $height);
$black = imagecolorallocate($image_p, 0, 0, 0);
$white = imagecolorallocate($image_p, 255, 255, 255);
$font_size = 14;
imagestring($image_p, $font_size, 5, 5, $text, $white);
imagejpeg($image_p, null, 80);
?>
//Form
<form action="submit.php" method="post">
Comment: <textarea name="coment"></textarea><br>
Enter Code <img src="captcha.php"><input type="text" name="vercode" /><br>
<input type="submit" name="Submit" value="Submit" />
</form>
//Submit:
<?php
session_start();
if ($_POST["vercode"] != $_SESSION["vercode"] OR $_SESSION["vercode"]=='') {
echo '<strong>Incorrect verification code.</strong><br>';
} else {
// add form data processing code here
echo '<strong>Verification successful.</strong><br>';
};
?>