You cannot delete your Pogo account.
Their official response:
“There is no way for players to delete a Screen Name from Pogo.”
“We do recycle Screen Names after so many months of inactivity, so if you simply stop using that Screen Name, it will eventually be wiped from our database. We recommend simply using the Screen Name you prefer, and keep the passwords for any other Screen Names you have to yourself; that way no one else can get a hold of them and cause trouble.”