I don't know if you can call it a hack
When ever a user add a new post or add a replay the ubb tack him to the old post with out modification
This hack solve this
In public_common.pl search for
code:</font><hr />
<hr /></blockquote>add after it
<blockquote><font class="small">code:
[qb]
my $waw=rand(9999999999);
$waw="rand=".$waw.";";
$_[0]=~ s/ultimatebb.cgi?/ultimatebb.cgi?$waw/gi;
[/qb]