There are two troubleshooting steps to take when a User can see a List View that is only being shared with a Public Group, and that User is not a member of the Public Group.
Step 1: Check the User's Profile to ensure that they do NOT have 'Manage Public List Views' permission. If this is checked, be sure to uncheck this setting:
If this does not resolve the issue, move on to troubleshooting Step 2:
Does your organization use Roles? If so, navigate to Setup > Administration Setup > Manage Users > Public Groups. Click on the Public Group name. Ensure that 'Grant Access Using Hierarchies' is NOT checked: