Originally posted by Graeme:

OK, I'll bite here..

(HOW LONG has it been since I posted at UBBDev??? )

Try:
in ubb_new_reply.cgi (circa line 403 in 6.3.1)
# grab last post
@rev_this_topic = reverse(@this_topic);
@last_post = split (/||/, $rev_this_topic[0]);
$last_post_num = $last_post[1];
$last_post_num++;

ADD right AFTER it:
if ($username eq $lastpost[2]) {
&StandardHTML("You cannot reply to this thread again until someone else does.");
}

Should be all you need. (NOTE: StandardHTML calls exit internally so you don't need it again)

(I tried to make it obvious enough over at community without spelling it out.. )

Graeme