way to control appstream fleet hostname

There is no way to assign a specific name to AppStream 2.0 computer objects. The objects have a specific description attribute which can be used for cleanup processes. The documentation can be found here: https://docs.aws.amazon.com/appstream2/latest/developerguide/active-directory-admin.html#active-directory-identify-objects.

Additionally it is a best practice to use a dedicated OU or sub-OU for your AppStream instances to allow for easy identification and AppStream specific group policy objects.

You can't specify the hostnames for AppStream 2.0.

As a best practice, specify a separate OU for AppStream 2.0 computer objects. https://docs.aws.amazon.com/appstream2/latest/developerguide/active-directory-admin.html#active-directory-oudn

AppStream 2.0 does not delete computer objects. You can clean them up on a periodic basis like you would other computer objects. https://docs.aws.amazon.com/appstream2/latest/developerguide/active-directory-admin.html#active-directory-identify-objects