Basic answer:
You can't. All AB posts are normally kept available for others (who might have similar questions) to read.
More complicated answer:
If you've got a really good reason why you'd like to see your post deleted, try a begging message to AB's editor. (Use the 'Contact Us' link at the foot of this page).
Chris
PS: While I try to provide 100% accuracy in my responses, there are one or two posts I'd rather weren't preserved. However I'm prepared to accept that the nature of the internet is that there's no going back on some of the things I've posted. Even if I got the editor to delete certain posts, there are other sites which which might still retain them among their archives.