It's possible. If you're just using the standard post islands, then what you'll need to do is get the list of TOPIC IDs that you want to exclude. You then need to edit the cache_builders/post_island.php file. Look for a query around line 70 or so. You'll see a portion that has the following code:

Code
        and t1.TOPIC_LAST_REPLY_TIME > $one_month
        and t1.TOPIC_STATUS <> 'M'

Right after that, add the following. A comma delimited list of your TOPIC IDs that you want to exclude. I'm using 2,4,6,8 as an example.

Code
and t1.TOPIC_ID NOT IN (2,4,6,8)