We probably aren't going to change this, but we can provide an option which explains how to disable their account on the instance itself, and maybe pull the instance account state onto the page so it immediately reflects the status, and maaaaaybe give you an action shortcut from the admin panel.

In particular:

• Member detail page should make it clear how to disable them.
• You should be able to cancel invites which haven't been accepted.
• Detail page should show disabled status and things like their account name on the instance.
• Main page should show total number of members and break their status down (invited, active, disabled).

This should get fixed, too:

Call to a member function getUsername() on a non-object
/core/lib/instances/src/applications/instances/controller/InstancesMemberViewController.php:32

That's like 90% of it but I want to let you cancel invites (if you're quick enough) and provide a hint for disabling users (if you aren't).