Testing shows the latest script is duplicating all the posts after each use.. (so the second time it does it twice, third time three times etc.)

Not sure why, will look into it and hopefully fix it.

Edit: And fixed. Problem with divs and class names, sorted now. Please use this script instead:
Edit2: As pointed out by Hyper, I forgot to mention that when you put page numbers in the boxes, for the whole thread the start page should be 0 not 1.

var $filterbar = $( '<div><p style="display: inline;">Username: </p><input type="text" value="" id="username_box" style="display: inline; margin-right: 10px;"><p style="display: inline;">First Page: </p><input type="text" value="" id="startpage_box" style="display: inline; margin-right: 10px;"><p style="display: inline;">End Page: </p><input type="text" value="" id="endpage_box" style="display: inline; margin-right: 10px;"><a id="filter_go" style="display: inline;">Go</a></div>' );

$( ".nav_bar_bottom" ).prepend($filterbar);
$( "#filter_go" ).on("click", function () {
jQuery('.spot_h').remove()
var endpage = parseInt($("#endpage_box").val());
var startpage = parseInt($("#startpage_box").val());
for (var i = endpage; i > startpage; i--)
{
var threadURL = "/forum/general/gog_forum_mafia_27_gameshow_showdown_the_game/page" + i + " .spot_h"
if (!$(".page" + i + "posts").length)
{
var $newdiv = $( "<div class='page" + i + "posts'/>" )
}
$( ".list_h" ).prepend($newdiv)
$( ".page" + i + "posts").load(threadURL, function() {
$( ".b_u_name" )
.contents()
.filter(function() {
return this.nodeType === 3;
})
.wrap( "<a></a>" )
.end()
jQuery('.b_u_name a:not(:contains(' + $("#username_box").val() + '))').parents('.spot_h').remove();
})
}
});