The easiest way to get a previous user's data cleared off of your phone is to go to Settings>General>Reset and click on Reset all Settings. Here is a link that will explain what that will do:
Hope this helps!
I also have somebodies elses ID on my Iphone. I did what you said and when I went to the store I saw my ID but not this other ID. Do you have any suggestions where I can find this person's ID on my phone and delete it because at the moment I am unable to download any apps, all the time this person's ID keeps coming up.