[kh99]

You could do something like this: create a plugin using hook location postbit_display_complete and this code:

if ($this->registry->userinfo['userid'] == 1 && $post['userid'] == 2)
    $post['message'] = "<b>You do not have permission to view this post</b>";

(of course you'd change 1 and 2 to the actual user ids of the blocked and blocking users).
But there are other ways to see a post that aren't covered by this, for example if your forum is not private to members only then the user you're trying to block would only have to log out.