Cant delete iCloud

im having issues with deleting an icloud account as i bought an iphone of someone, i keep getting a message telling me to enter the password for the iCloud account.. the previous owner doesnt know the password for the account so it cant be accessed, but when i try to turn of find my iphone in the icloud settings everything is greyed out and i cant even sign out of the icloud account on the phone either! i am i 8.1.2