Edit: Based on comments, here's how to find an offending sharing reason. First, go to the case detail page, and click the sharing button:
Then, click on the Expand List button:
Finally, find the user in question and click "Why?" to learn why they have access:
You'll see a list of reasons why a user has access, such as the following:
You can see that, in this example, a user is an administrator, the owner of the record, and a sharing rule grants access. In order to "hide" this case from the specific user, I would need to remove Modify All Cases (make them a normal user), change ownership, and modify the sharing rule, or move the user to a new role. Your specific case will vary.
For example, it might be because of Implicit sharing (meaning, they own a related record that provides access), or it might be a Role and Subordinate rule, possibly on a related record, etc. An exhaustive list isn't really going to help you here. You need to read the sharing, understand why, then take the appropriate action to reverse the problem.
Sharing is cumulative, which means you'll have to take the opposite approach: set your OWD to Private, then build sharing rules so that people who need to see each other cases are in the same groups. If everyone except that one user needs to see each others' cases, you'll most likely want to move the lone user to a role all their own that is a sibling of the existing role, and set up the sharing rule to include that single user (if necessary) for the original role.
In other words:
- Parent Role
+- Original Role
+- Isolated Role
With a sharing rule as follows:
Members of Roles and Subordinates Parent Role shares with Original Role.
This sort of sharing change is bound to have lots of side effects, so make sure you check very carefully before deploying this to your org. In fact, I'd simulate it in a sandbox first, and test as various users and profiles in various roles to make sure nothing seems amiss.