I remembered where this setting is in the minefield that is forum settings! So have changed it for the buy&sell - will do it for the rest of the forum shortly
You may need to sign in and out to see a cross at the top right of the thread - that will delete the post....BUT if there were posts made by other people on your your thread you'll need to ask me/admin to delete the whole thread as you'd just delete your post and leave the other posts sat there looking confused