A comment was raised at [[en:Wikipedia talk:Special:PrefixIndex#Inappropriate_default_search.3F]], that the default results of [[en:Special:PrefixIndex]] include obvious profane titles.
Being able to hide pages beginning with ! would solve it at that particular wiki, but a more general solution might be to simply not display anything when the page is initially shown, since the default results (blank prefix search) are unlikely to be helpful in its use as a search page. If it's possible to distinguish between searching on a blank prefix and not having yet searched on anything, that would neatly solve the problem. Not displaying results even for a deliberately blank search would be no great loss either as the same results can be got via [[Special:AllPages]] or [[Special:AllPages/!]].