Material Group

Hi ALL

I created a material group for service and then i removed (007)and created a new group(ser) .but whenever i created a any po and give Item category as D it first pick deletd material group (007)flows as a default then i have remove and give new one(ser) Even in material group if i click and give f4 ,there is no that matl group(007).how to remove that(007)

regards

kumar