[_ck_]

It would be possible to do that via a plugin.

this might work, untested:

add_action('bb_head','close_long_topics');

function close_long_topics() {

global $topic;

if (is_topic() && $topic->topic_open===1 && $topic->topic_posts>299) {

bb_close_topic($topic->topic_id);

$topic->topic_open=0;

}

}