How long has it been since you deleted the sub-domain? It can take a little while for the A-Records related to the sub-domain to be deleted.
Sometimes, the A-Records seem to get 'stuck' and don't get removed automatically. In such cases you will need to contact support so they can remove them for you.
Save [color=#CC0000]$50[/color] on DreamHost plans using [color=#CC0000]PRICESLASH[/color] promo code (Click for DreamHost promo code details)