Hey tiphiee,
Thanks for using Apple Support Communities! I understand that you need to prevent someone from sharing their location with you. I definitely want to make sure you have control over that, and I am happy to share some information.
Based on the information you provided, it sounds like you'll need to remove them from Find My Friends. When you remove a friend, you stop following that person, and that person stops following you. Any location sharing that began in a Messages conversation is also stopped.
You can see how to remove them here: Find My Friends: Remove Friends
I hope that helps you take back control over location sharing! Cheers!