You can remove a user's access to a case by removing them as the Case worker and/or Case owner, depending on which role they have.
Steps
Select My Apps in the top menu
Select Cases
You arrive on Manage Cases
Find the case you want to remove someone from, e.g. by scrolling, using the page numbers or using the filters
Select that case
You arrive on the Case profile
Press Manage Case
To remove someone as the Case Owner (Case Creator), select a different user in that dropdown list
To remove someone as a Case Worker, delete their name from the Case Worker field
Press save