how to delete old account and credit card info by
Settings > Accounts > Google then select the account which you wish to remove. 

Settings > Accounts > Google >  select the account which you wish to remove > More  > Remove account. 

PS: You'll need to stop Syncing first.

On older versions (1.5 > 2.3.5) there is no way to remove an account, only by resetting the device to the factory settings.