@abirakundu23 HR Agent will not be able to create a case unless they select a valid subject person. There are two steps of case creation in workspace

Step 1: Select the subject person (Agent can skip the verification)

Step 2: In step 2, the Agent need to choose a valid subject person, they can't skip this step and an error will be shown if they try to enter a user which doesn't exist.

A nutshell a case can't be created unless you choose the right subject person.

Hope this helps.