Hi Shuang,

for audit purposes (all past actions are kept) we avoid deleting users, that’s why you don’t have this option.

If you think you really need to delete one or several, for example you added 30 people by mistake and they never did anything, contact support.

On your related question:

As long as you have the permission to Define Application Security (e.g. are an admin in an application) you can change the Application Owner and claim it your own in the security tab.

Since data can be hidden from Workspace / Security Admins (workspace level roles) it’s important that this permission is set in the application itself (admin of the application).

While deactivating a user, you also get the option to set a new owner for all applications that were owned by this user, so nothing is lost.