UBB 7.3 kinda has a bug that I was happy with, you can't quote so deep.
I made some code that checks for the quote level, and gives an error if you quote more then 3 levels deep.
open /scripts/addpost.inc.php
find:
// -------------------------------------
// Make sure there is a subject and body
add above:
// ---------------------
// Check maximum quote level
unset($total);
preg_match_all('%(\[quote=.+?\]|\[quote\]|\[/quote\])%', $Body, $result, PREG_PATTERN_ORDER);
for ($i = 0; $i < count($result[0]); $i++) {
$total .= $result[0][$i];
}
if (preg_match('%(\[/quote\]){4,}%', $total)) {
$html -> not_right($ubbt_lang['MAX_QUOTE']);
};
open /languages/english/addpost.php (plus every other language you might have)
find
$ubbt_lang['VIEW_POST'] = "View your post.";
add below:
$ubbt_lang['MAX_QUOTE'] = "For readability you can only quote messages 3 levels deep. Please edit your message and try again.";
The 3 levels deep is hardcoded in the regex, so if you want to change it, change the regex.
Does your forum have issues with it? I understand there's a bug, but does your users actually exploit it? 
over 5-6 quotes deep make little to no sense to me conversation wise.
