Restrict case to be editable or processed only by the current owner / team

Hi,

I have a require such that, case can be edited (or proceed the workflow) only by the owner. ie, if a case is assigned by user A to user B, then user A can view that case but cannot act on the case.

The security role for user A and user B has Organization level privilege to create & read and user level privilege for edit, append & append to. However, even if the case is assigned by user A to user B, user A can still edit the case even though the new owner is user B. Why so?

Highly appreciate any help. Thanks!