You could set the user to be blocked and then notify the admin by email.
Around line 641 in webloginpe.class.php change from :
$newUserAttr = "INSERT INTO ".$web_user_attributes.
" (internalKey, fullname, email, phone, mobilephone, dob, gender, country, state, zip, fax, photo, comment) VALUES".
" ('".$key."', '".$fullname."', '".$email."', '".$phone."', '".$mobilephone."', '".$dob."', '".$gender."', '".$country."', '".$state."', '".$zip."', '".$fax."', '".$photo."', '".$comment."')";
$insertUserAttr = $modx->db->query($newUserAttr);
to :
$newUserAttr = "INSERT INTO ".$web_user_attributes.
" (internalKey, fullname, email, phone, mobilephone, dob, gender, country, state, zip, fax, photo, comment, blocked) VALUES".
" ('".$key."', '".$fullname."', '".$email."', '".$phone."', '".$mobilephone."', '".$dob."', '".$gender."', '".$country."', '".$state."', '".$zip."', '".$fax."', '".$photo."', '".$comment."', '1')";
$insertUserAttr = $modx->db->query($newUserAttr);