As V-man explained, this configuration setting will keep your site users from scrolling back to previous time displays in the past, so it sounds like all you need to add is the ability to disable past events on the current views.
Here are some hacks to do this to satisfy your request ... keep in mind these are just hacks to eliminate past dates on the current display. The user could still access these events directly. A decision would have to be made to change the core functionality before a more complete solution could be provided. So, keep these hacks around, as you may have to re-apply them on your next upgrade.
But, they seem to work fine.
For flyer view, in "jcalpro.php", around line 673, change the number "1" to "$day" as indicated in red below:
echo "<img class='miniarrow' src='$mosConfig_live_site/components/com_jcalpro/images/mini_arrowright.gif' border='0' alt='$date' />";
echo "</a> </td>";
echo "</tr>\n";
for ($i=$day;$i<=$nr;$i++)
{
$date_stamp = mktime(0,0,0,$month,$i,$year);
$events = get_events($date_stamp,$CONFIG_EXT['show_recurrent_events'],$CONFIG_EXT['show_overlapping_recurrences_flatview']);
For the monthly view, in "jcalpro.php" around line 523, change the lines in red below to look like the following:
// 'existing' days in month
for ($i=1;$i<=$nr;$i++) {
$date_stamp = mktime(0,0,0,$month,$i,$year);
$events = array();
if( $i>=$day ) $events = get_events($date_stamp,$CONFIG_EXT['show_recurrent_events'],$CONFIG_EXT['show_overlapping_recurrences_monthlyview']);
//$events = sort_events($events, $event_stack, $date_stamp);
For the category view, in "jcalpro.php" around line 836, change the line in red to look like the following:
$day_pattern = sprintf("%04d%02d%02d",$today['year'],$today['month'],$today['day']); // day pattern: 20041231 for 'December 31, 2004'
$query = "SELECT * FROM ".$CONFIG_EXT['TABLE_EVENTS'] . " AS e WHERE (DATE_FORMAT(e.start_date,'%Y%m%d') >= $day_pattern OR DATE_FORMAT(e.end_date,'%Y%m%d') >= $day_pattern OR DATE_FORMAT(e.recur_until,'%Y%m%d') >= $day_pattern) AND cat = '$cat_id' AND approved = 1 AND published = 1 ORDER BY title";
$result = extcal_db_query($query);