Hi Mary, We covered this in some depth a few months ago when we changed from the previous mailing list format to the forum format. You can't "delete" an account - you can stop participating, and cancel your account (so you can't login, can't get notifications, won't get charged again if you are a paying member, etc).
Orchid is, always was, and will forever remain, a public forum. Once you post, that post is publicly available and it remains archived in perpetuity. One can always cease participating, but the content you've already contributed remains available to the public. Forever.
Orchid has always operated in this manner - the discussions have been archived since 1996.
A member can modify a post shortly after posting, just to make a quick correction. They can even remove a post, again within a relatively short window, and that post may or may not be deleted (a moderator needs to approve a deletion).