This bulletin board permits posts to be edited (i.e. changed), both by the poster him/herself, or by moderators.
Sometimes posters want to remove an entire posting or set of postings. We would like to discourage this, since other posters may have debated/ argued/ made some effort, and to leave their postings orphaned without the surrounding messages disrupts the conversational flow and implies, in effect, that their work was an irrelevance and not worth doing.
The policy of this site is to encourage incisive debate; if you make a mistake, acknowledge it, don't remove the evidence. Other people no doubt make the same mistake, and would benefit from reading a refutation.