How to delete the preowner icloud account without knowing the password?

It cannot be done. You need to have the previous owner remove the device from their account.

II'm in the same situation. Purchased a refurbished IPhone 5. I had to do a factory reset and when I reset it again, itunes tells me that the phone is linked to someone else. Called the company I bought the phone from and they say iTunes can do a master reset to link my iTunes account to the phone. Have any of you heard of this being possible?