To stop an account from following, or interacting with you, all you need to do is block them. You will see the option to block an account in the top right of their profile.
When blocking an account you will also be given the option to remove reposts, likes and comments made by that person on your account.
Please be aware that if that member tries to follow you, they will be notified that your account has blocked them.
You can view who you have blocked in your Privacy Settings page here and choose to unblock them, if you wish. You can also unblock a user directly from their profile page.