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Excerpted from FAQ's. Bottom line seems to be that possibly the administrators may have turned off the option to delete 
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"Editing and Deleting your Posts
Can I edit or delete my posts?
If you have registered and are logged in, you may be able to edit and delete your posts (although the administrator may have turned off this option). Your ability to edit your posts may be time-limited, depending on how the administrator has set up the forum.
To edit or delete your posts, click the Edit button by the particular post. If your post was the first in the thread, then deleting it may remove the entire thread.
Once you've made your modifications, a note may appear to inform other users that you have edited your post.
If the edit time and date appear as a link you can click on this to see the differences between the original and edited versions, or between edits if there have been multiple edits.
Can others edit my posts?
Administrators and moderators may also edit your messages. If they do, there may not be a note telling other users that the post was modified."