"Enemy System" ;-)

if (isloggedin()) {
    if ($_SESSION['user']->getGUID() != $vars['entity']->getGUID()) {
        echo "<p class=\"user_menu_block_user\">"
        . "<a href=\"" // we want to zap this user fro all our content ;-)
        . "{$vars['url']}action/friends/userblock?usertoblock={$vars['entity']->getGUID()}\">"
        . elgg_echo("userblock:zaphim")
        . "</a></p>";
}
. . .

// and later
$Block = get_metadata_byname($_SESSION['user']->getGUID(), 'BLOCKED');
$Blocked = $Block->value; // bool
if ($Blocked) // we zapped him ?
header("Location: {$CONFIG->url}" ); // do not allow access -> fwd to site home page