se_bbpm_spam_check.php

<?php

function se_bbpm_add_spam_check() {

	/**
	  * add spam check for users with bbp_spectator and bbp_participant roles
	  * Add more roles in the $targetRoles line or replace existing ones to match
	  * your targeted users
	  */

	$targetRoles = array( 'bbp_participant', 'bbp_spectator', 'administrator'/*debugging*/ );

	$current_user = wp_get_current_user();

	foreach( $targetRoles as $role ) {
		if( in_array( $role, $current_user->roles ) )
			return true;
	}

	return false;
}

add_action('bbpm_conversation_form_additional_fields', function() {

	/**
	  * Follows is just an example on how to implement a simple captcha,
	  * You can set the answer in a cookie or database option and fetch
	  * it with a unique key, or store it somewhere else, use honeypot and
	  * spam hidden traps, reCaptcha, or simply a captcha image..
	  */

	if( ! se_bbpm_add_spam_check() )
		return; // not enabled for this user

	$rand1 = rand(5,20); // generate random number
	$rand2 = rand(1,9); // generate random number

	$operations = array(
		'+' => 'plus',
		'-' => 'minus'
		// add more as you wish, be careful with floats while *|/
	);

	$operationRand = array_rand($operations); // get a random operation from operations list

	?>

		<p>
			<label>
				<strong style="display: inline;">Spam test:</strong> What's <?php echo $rand1 . ' ' . $operations[$operationRand] . ' ' . $rand2; ?>? <br/>
				<input type="text" name="capCheck[value]" />
				<input type="hidden" name="capCheck[operation]" value="<?php echo $operations[$operationRand]; ?>" />
				<input type="hidden" name="capCheck[a]" value="<?php echo $rand1; ?>" />
				<input type="hidden" name="capCheck[b]" value="<?php echo $rand2; ?>" />
			</label>
		</p>

	<?php

});

add_filter('bbpm_bail_sending_message', function( $bail ) {

	echo '11';

	if( ! se_bbpm_add_spam_check() )
		return $bail; // not for this user

	if( ! isset( $_POST['capCheck'] ) ) {
		return true; // nothing set
	}

	if( ! is_array( $_POST['capCheck'] ) )
		return true; // no data is set

	$operations = array(
		'+' => 'plus',
		'-' => 'minus'
		// add more as you wish
	);

	$op = $_POST['capCheck']['operation']; // which operation used
	$a = (int) $_POST['capCheck']['a']; // element a
	$b = (int) $_POST['capCheck']['b']; // element b
	$ans = false;

	if( "minus" == $op ) { $ans = abs( $a - $b ); }
	else if( "plus" == $op ) { $ans = abs( $a + $b ); }
	// add more ..
	else { return true; }

	if( $ans == (int) $_POST['capCheck']['value'] ) { // drop (int) when working with non-integer strings
		return false; // continue sending message
	} else {
		return true;
	}
	
	return $bail; // can be true|false

});