How can I remove a person from accessing my case?

You can remove the person to access your files after opening particular file and click on the 'edit' icon in the 'Assigned to file' window and confirm removal by the 'Remove' button.

You can also remove the assigned person to access all your future files by deleting such option in your user account. Removing the person from your user account however does not automatically remove such person from the existing files, for such cases you need to remove it manually as described above.