To unshare a to-do list with the user (expell the user from a list), simply open the list details and tap on the user to be removed. And confirm.
Or locate the user in the 
Sharing tab and untick the shared list.
The user will be removed from the list, and a copy of the list will be permanently deleted from the user's account. All actual invitation links will be reset too.
Please be aware that only invited users are eligible to be expelled from the list. The list owner (creator) may not be expelled.