You can't it seems. You did ask the same question a year ago …
Years back, there was this to go by, but I’m not really sure it still stands (haven’t seen any case where it has been applied to since I’ve been around):
Deleting accounts doesn't mess up any threads, like
this thread. It just changes to anonymous, with no profile information. Post stays intact.
As you can see here, I only change the username to "Anonymous" on request.
I guess your question is intended for another account. If not, with your current one you can simply try to delete each post. Nevertheless, Google (cache) and archive sites are bummers, and possibly traces will of your posts may still be retrievable even if it were feasible here.
