1.Enter your email address and password at icloud.com/find
2.Click on 'all devices' in the top centre
3.Click the iPhone in question in the list
4.Then click 'Remove from account' and confirm your choice.
5.Then open this list again by clicking on 'all devices'
6.Click on the 'X' next to the phone in question
7.Confirm that you want to delete the phone