I don't think the Web UI provides a simple way of doing this, but you can easily remove those posts directly from the database. All posts under moderation have a type ending with the suffix "_QUEUED," so you can use that as the filter for a DELETE query.
DELETE FROM qa_posts WHERE type LIKE "%_QUEUED";
If you want only a particular type of post removed (e.g. only questions), change the filter to that type (e.g. type="Q_QUEUED").
Note that removing posts under moderation directly from the database will not automatically reset the counter for those posts, so you'll have to do that yourself as well.
UPDATE qa_options SET content=0 WHERE title="cache_queuedcount";