Removing a collaborator from a case removes their access to any content or forms that were shared with them from that case.
From the case overview page, find the Collaborators panel and click Manage collaborators above the panel.
On the right-hand side of the page that appears, find the collaborator you want to remove and click Remove from case.
Once removed, the collaborator will not appear on the case or be able to access the content or forms you shared with them from that case.