Deactivating them should actually do what you need, I think. Topics are stored and referenced by topic_id, not by topic. So you'll be able to create new topics with the same names later without any problem, should you need to do that.
If you added thousands of test messages to these test topics, you might want to do a little careful playing around in psql to delete them and then vacuum your database, something like
psql -U acs acs
select topic_id, topic from bboard_topics; -- Look up topic id(s)
delete from bboard where topic_id in (2,4,7); -- Delete msgs topic
vacuum analyze bboard; -- really clean out deleted messages
(replacing (2,4,7) with the set of topic_ids of the bboard you need to delete mesages from). If you only had a few messages in there anyway, though, I'd think that there's no need -- just deactivate the unwanted topics.
