If you defined an user as "foe" in the User Control Panel, his or her messages are automatically hidden by default (as soon as you log in your account).
In Viewtopic pages, however, you still have the possibility to read the hidden content if you want (by clicking on Display this post). This is a good feature : although you defined someone as foe, you can sometimes feel the need to read his or her post, without changing the foes list.
Oddly, that possibility doesn't appear neither in search results, nor in post/topic review. Only in topic pages (viewtopic.php). It doesn't make sense.