The Jargon File (version 4.4.7, 29 Dec 2003):

spoiler space


    [also spoiler spoo or spoiler protection] A screenful of blank or spacer
    lines deliberately inserted in a message following a spoiler warning, so
    the actual spoiler can't be seen without hitting a key. Formfeeds used to
    be used for this, but are now rare because so many people read news through
    Web interfaces on which they have no good interpretation.