you will need to contact the previous owner and have them login to the icloud account and remove the association, either on the phone itself, or via either www.icloud.com or appleid.apple.com.
beyond that there is no permanent solution.
yes, you will find services and software offering to remove icloud accounts, it's all BS. these "solutions" involve jailbreaking the phone then installing a "bypass". but the phone will not behave like a normal phone (if i'm misstating some of these items, i welcome correction as a am not a jailbreaking expert). you will likely have issues with downloading and installing apps, you may have issues trying to use the only on you cellular service account, you cannot do auto upgrades the IOS, your device becomes more vulnerable to malware/hacking, battery drain/usage increases dramatically, it makes the overall IOS environment less stable meaning more crashes, and if you do upgrade the IOS or reset the phone, you have to re-jailbreak it again.
using jailbreaking to "fix" an icloud account is like (you may be too young to actually relate to this) watching TV before they had remote controls. you want to change the channel or change the volume or adjust the rabbit ears, you had to get out out of your chair, walk to the tv and do so manually - every time. sure it all worked, but it was horribly inconvenient compared to what a modern day TV remote can do for you.
if anyone has a better analogy, i welcome input on that regard as well. :)
bottom line, the only way to properly remove it is to have the icloud account owner do it
