I do clean out the test and the chit chat boards every now and then, but not the other boards. You never know when an old topic will be helpful to someone and people use our search function a lot when they are looking for help with a topic and don't want to post.
If you need to have a post deleted for some reason you can contact a moderator or click on the "Report to moderator" link at the bottom of each post. You will then have a chance to type in the reason you want it deleted.