I imagine the simplest solution is to simply edit the posting down down to something like "deleted", plus maybe a brief explanatory note if you like. That way there's still a posting, but essentially nothing in it.
A related question, while on topic -- something vaguely rings a bell in my mind that there's a time limit after which you can no longer edit. But it's fairly generous, something like 30 days, so "afterthoughts" a day or two later aren't a problem. Can someone confirm or correct my often-faulty memory?